Sources of Specimens
There are a great
many clinical laboratory tests, and they are performed
on blood, urine, sputum, and other body fluids, and
occasionally feces (see
table 4.1). Tests can be performed on whole blood
(to which an anticoagulant has been added to keep it
from clotting), plasma (the fluid that remains
when whole blood is centrifuged to remove the suspended
red and white blood cells), or serum (the clear
fluid that separates from whole blood that has clotted).
Many of the
substances that are measured in blood can also be
analyzed in urine or other body fluids, although the
results will have different reference (normal) ranges.
For example, glucose, a form of sugar, is not normally
found in urine, but it is in blood, where it is about
twice as concentrated as it is in cerebrospinal fluid
(the fluid that surrounds the brain and spinal cord).
urine, sputum, and cerebrospinal fluid, other body
fluids commonly examined in clinical laboratories are
bronchial or pleural washings (fluids from the lungs and
bronchial tubes), gastric or stomach aspirations, serous
(or peritoneal) fluids from the abdominal cavity, and
joint fluids. The various methods for obtaining these
various body fluids are described below.
Blood is most commonly drawn via venipuncture or finger
Blood is usually drawn from a vein on the inside of the
elbow. If your doctor orders this venipuncture
procedure, the nurse or technician will first wrap a
tourniquet (usually a rubber hose) around your arm above
your elbow to compress the blood vessels and limit the
flow of blood in the veins that would normally return to
the heart. You will then be asked to make a fist, which
will make your vein stand out more prominently. The skin
on the inside of your elbow will be cleaned with a swab
or piece of cotton dampened with alcohol, and a sterile
needle inserted into your vein. A coupling device
attached to the needle allows blood to be drawn
automatically by vacuum pressure into rubber-stoppered
tubes. When a tube is filled, it can be removed and
additional ones attached, depending on the amount of
The needle is
then withdrawn from the vein, and the tourniquet removed
if it hasn't been removed earlier. (Needles are always
disposed of after one use so that there is no chance of
spreading infection.) The entire procedure generally
takes less than five minutes. You will be told to apply
pressure to the puncture site with a piece of cotton for
a few minutes. A small bandage may be placed over the
site; this bandage can be removed in less than an hour.
You should refrain from using your arm to carry heavy
loads or do strenuous chores for about half an hour.
If for any reason
blood cannot be drawn from an arm vein, the one inside
your wrist or on the back of your hand can be used
instead. For hospitalized patients, blood at times is
obtained from the intravenous tubing used to deliver
fluids directly into a patient's vein. Some tests are
done on blood drawn from an artery instead of a vein,
but these are rare. Because of the increased risk of
bleeding, however, arterial blood is drawn by a doctor.
blood drawing is minimal. You may be instructed to
refrain from eating or drinking anything except water
for about eight hours before blood is drawn. These
so-called fasting specimens guard against interference
from the elements in food or liquids that may cause
inaccurate test results. Blood glucose and triglycerides
(a constituent of fats) are examples of tests that
should ordinarily be done on fasting specimens. For
many, if not most, tests, however, nonfasting (random)
specimens are fine.
The amount of
blood that will be drawn depends on the total amount
needed for a particular test, as well as the amount
needed in each tube to mix with an anticoagulant or
preservative to achieve the desired effect. While one or
more tubes of whole blood may be drawn, usually only a
fraction of it is needed, but that fraction may come
from the whole blood, plasma, or serum. To put this in
perspective, your body contains about 100 ounces, or 3
quarts, of circulating blood. A typical tube contains
only about 1/3 of an ounce. A drop of blood, serum, or
plasma is often enough to do one test, and sometimes
many, on an automated instrument. This drop is equal to
about 2/1000 of an ouncea trivial amount compared to
the total amount available in your body.
If an even smaller amount of blood is needed (to check
for anemia if you are planning to donate blood, for
example) or if your veins are too small or too fragile
for a venipuncture, blood can be obtained by sticking a
finger with a small, sharp blade. This is frequently
done in children. An earlobe or a heel (especially in
newborns or infants) can also be used.
For a finger
stick, the nurse or technician will wash your skin with
alcohol and then make a quick prick with the blade
designed to obtain blood from capillarieshair-sized
vessels that connect the smallest of veins to the
smallest of arteries. He or she will then gently squeeze
your finger to produce drops of blood that are gathered
into micropipettes (tiny glass or plastic straws) or
very tiny tubes. Because capillaries are so small, they
usually produce only enough blood for a few tests, and
the blood flow quickly ceases. There are no precautions
to take after the test; you may not even need a bandage.
Urine for analysis can be obtained in several different
ways. The most common method is a random (also
called a "spot" specimen that is used
for the standard chemical and microscopic urinalysis. It
simply requires you to urinate into a cup, jar, bottle,
or tube. The container must be thoroughly clean and dry,
but it needn't be sterile. If the specimen is not
directly examined, or sent within a few hours to a
laboratory, it should be refrigerated.
A clean catch
specimen requires that you thoroughly clean your
external genital area with a mild soap and water and
then dry off the area before urinating into a clean, dry
container. This is because skin naturally contains
bacteria that could obscure bacteria from your urinary
tract or falsely indicate an infection. Your doctor will
commonly ask for a clean catch specimen if a kidney or
bladder infection is suspected.
If a sterile
urine specimen is needed to identify a specific
bacterium, a doctor or nurse will obtain it by
catheterization. While you are lying down, a
cathetera thin, flexible tubeis inserted into the
outer opening of your urethra, the tube through which
urine from your bladder leaves your body. Urine is then
drained into a sterile specimen container. This
technique is often used for patients who cannot urinate
voluntarily. If that is the case, the catheter may be
left in place.
doctor may request a timed test, which measures
the quantity of a substance excreted in the urine over a
period of timetypically 24 hours, but occasionally
two, six, or 12 hours. For a 24-hour urine test,
you begin by voiding and discarding the first specimen.
This is because the substance being measured has to be
estimated over an exact period of time. Including the
first specimen, which has been building up in your
bladder over an unspecified amount of time, will throw
off the measurement of the substance (such as a hormone)
being tested. After discarding the first specimen, you
collect the rest of the urine you void over the
specified time period in a clean plastic jug, which may
contain a preservative (for specific instructions, see
chapter 12. In the laboratory, the total volume of urine
is measured and an aliquot, or sample of the total
volume, is used for the analysis.
Other Specimens. Sputum (phlegm) is the product
of a deep cough and can be collected directly into a
clean, widemouthed plastic or glass specimen container.
Sometimes you may be asked to cough directly into a
Petri dish. This round, shallow glass or plastic dish
with a cover contains a gel-like substance, or medium,
in which bacteria will grow.
Other body fluids
(cerebrospinal, pleural, abdominal, or joint) are
obtained by aspiration. After a local anesthetic is
injected or applied to your skin, a fine needle is
injected into the appropriate body cavity or joint, and
a small amount of fluid is aspirated (withdrawn). The
fluid can then be examined for its microscopic cells or
chemical constituents, or cultured for infectious
need to be collected directly into a clean, dry
cardboard or plastic container.
Clinical Laboratory Tests
laboratories are typically divided into several
specialty laboratories, which may include
hematology, clinical chemistry, immunology and
serology, microbiology, and the blood bank. The
tests described in this table are arranged
according to the laboratory in which they are
usually performed (indicated in boldface).
This laboratory examines the formed or cellular
elements of blood, which include red blood
cells, white blood cells, and platelets (cells
necessary for clotting); the amounts of clotting
factors; and the types and amounts of
hemoglobin, the red pigment in red blood cells.
Such disorders as anemias, hemophilia and other
blood-clotting disorders, and leukemias are
first diagnosed and then monitored in this
laboratory. Organizationally, the urinalysis
laboratory is often grouped with hematology.
Urine is examined both chemically and
Shows/What It's Used For
blood count (CBC) (includes white blood cell
count, red blood cell count, hematocrit, red
blood cell morphology and indices) and
differential (the proportion of the various
types of white blood cells in the blood)
anemias, some cancers such as leukemias and
lymphomas; evaluates blood loss and response to
infection. Sometimes only a part of the CBC is
performed (eg, white blood cell count or
hematocrit). Often used as a general screening
test before surgery or as a part of a routine
diagnoses, and monitors bleeding and coagulation
time (PT), partial thromboplastin time (PTT),
and specific clotting factor assays
anticoagulation therapy (PT, PTT); evaluates
bleeding and coagulation disorders such as
red blood cell production.
urinalysis (includes color; pH; specific
gravity; turbidity; chemical analysis for occult
blood, protein, ketones, and glucose; and
microscopic examination of sediment for red
blood cells, white blood cells, bacteria,
crystals, and casts)
kidney and bladder infections and other
diseases, certain metabolic and systemic
diseases, dehydration, and urinary tract
chemistry: This laboratory is concerned
primarily with measuring the amounts or
concentrations of various chemical constituents
of blood, and less often with simply identifying
their presence. The scope of clinical chemistry
is very broad, and many different tests can be
done to assess various substances found in blood
or urine. The major ones are listed below.
Levels of enzyme activity in blood help
determine which organs are damaged or diseased
and to what extent. When organs or tissues are
damaged, enzymes leak out into the blood. The
following are examples of enzymes produced by
Shows/What It's Used For
marker for acute myocardial infarction (heart
attack). Also present in skeletal muscle. CK-NB
is a form of CK that is mostly found in the
heart muscle and provides more specific
information about heart damage.
marker for acute myocardial infarction. Present
in all organs and also released into blood in
disorders of liver, kidneys, red blood cells,
and muscle. Isoenzymes, or forms of the enzyme
that are specific for different organs, can help
pinpoint the source of LDH elevations.
aminotransferase (AAT, SGOT) and aspartate
aminotransferase (AST, SGPT)
in many types of liver disorders including
hepatitis. May also be abnormal with damage to
several other organs or tissues.
in obstructive liver disease, in which excretion
of bile by the liver is impaired. The causes
include gallstones, tumors, and some forms and
stages of hepatitis. Also elevated in bone
disease, including Paget's disease, vitamin D
deficiency (rickets), hyperparathyroidism, and
cancer that has metastasized to the bone.
Because their bones are growing, healthy
children have higher values than adults.
in inflammation of the pancreas (pancreatitis),
and less often in cancer of the pancreas.
Hormone levels in the blood are used to evaluate
the function of various endocrine glands and can
indicate hyper- (over) and hypo- (under)
Shows/What It's Used For
gland: elevated with uncommon tumor of adrenal
gland that can cause hypertension.
(T4), TSH, T4 indices
LH, GH (growth hormone), TSH
gland function; directly relates to function of
adrenal glands, sex glands, and thyroid gland.
and lipoproteins: These help evaluate risk
of coronary heart disease. They are also
sometimes used as markers of liver disease and
Shows/What It's Used For
but not absolute marker of coronary heart
low-density lipoprotein cholesterol (HDL and LDL)
of cholesterol that provides better estimate of
risk than does total cholesterol alone.
cholesterol, used to evaluate coronary heart
These reflect metabolic and nutritional status
in a wide variety of disorders, and
overproduction in some cancers.
Shows/What It's Used For
some forms of liver and kidney disease, and in
in some chronic infectious and inflammatory
illnesses and some blood cancers. This test
includes globulins or antibodies produced by the
body in response to infections and allergens.
Abnormal globulins can be detected in multiple
myeloma and related disorders. Protein
electrophoresis fractionates serum proteins into
various classes, which allows for more specific
These tests help to identify and evaluate such
metabolic disorders as acidosis, alkalosis,
malnutrition, dehydration, and various bone,
kidney, and endocrine gland disorders. Results
of these tests are nonspecific, and can be
abnormal in a variety of disorders too numerous
to include here (although a few are listed in
table 4.2). Electrolytes are also affected by
megadoses of vitamins and minerals, and by such
drugs as diuretics and antacids.
abnormalities in electrolytes can have
important, and sometimes urgent, medical
consequences and therefore require rapid
intervention and treatment. In seriously ill,
hospitalized patients, these tests may need to
be monitored frequently, so that any
abnormalities can be quickly corrected.
which may vary individually or in concert with
each other, are often measured in a group. The
term usually refers to sodium, potassium,
chloride, and bicarbonate (some labs report CO2
instead), but may also include calcium,
phosphorus, and magnesium.
glucose (blood sugar): The glucose tolerance
test is used to assess the handling of glucose
by the body. In one form of diabetes (Type 1),
it reflects insulin release by the pancreas. In
the other (Type 2), it reflects insulin
sensitivity of various body tissues, such as
liver and muscle. It is also used to assess low
blood glucose, although less frequently.
Shows/What It's Used For
and monitors diabetes mellitus, evaluates and
diagnoses other disorders of carbohydrate
metabolism, and diagnoses hypoglycemia (low
test that allows more specific diagnosis of
diabetes mellitus after finding elevated fasting
blood glucose levels.
Shows/What It's Used For
urea nitrogen) and creatinine
these metabolic waste products eliminated by the
kidneys. Elevated when kidney filtration
function is impaired and in dehydration.
these metabolic waste products derived from
proteins. Elevated in gout, in some forms of
kidney disease, and with excessive tissue
and trace elements: Vitamin and trace
element (mineral) levels can indicate
deficiencies that can be responsible for anemias
and nervous system and metabolic disorders, as
well as excess due to industrial or
environmental exposure, which can result in
symptoms and signs of toxicity or poisoning.
(For testing for substance abuse, see chapter
Shows/What It's Used For
anemia and neurological symptoms.
vitamins (thiamine, C)
rarely measured to evaluate various unexplained
symptoms consistent with vitamin deficiency.
anemia and/or neurological symptoms. Screening
of infants and young children for environmental
neurological symptoms; suspicion of poisoning.
and serology: This laboratory is involved
with identifying antibodies (proteins produced
in the body in response to an antigen, which can
be an infectious agent, virus, toxin, or other
foreign substance) or in the diagnosis of
autoimmune diseases (antibodies against the
body's own tissues) and immunodeficiency states
(indicative of an underactive immune system).
Shows/What It's Used For
to infectious agents
to various infectious agents.
antibodies, complement, autoantibodies
diseases, especially systemic lupus
infection, acute rheumatic fever, acute
glomerulonephritis, and other streptococcal
agglutinins and monospot test
leukocyte antigen (HLA)
with disease syndromes, paternity exclusion
testing, and transplantation donor and recipient
matching (tissue typing).
states and certain malignancies, especially
classification and diagnosis of rheumatoid
(Venereal Disease Research Laboratory)
if positive, must be confirmed with a
more specific test.
This laboratory is responsible for the diagnosis
of infections by isolating and identifying
infectious agents in blood, urine, sputum,
feces, cerebrospinal fluid, and other body
fluids, and for testing for their sensitivity to
various antibiotics used to treat these
infections. Bacteria, viruses, parasites, and
fungi are identified by using such techniques as
staining, microscopic examination, and chemical,
immunological, and genetic tests.
Shows/What It's Used For
bacteria that cause tuberculosis and monitors
or "blood poisoning" (bacterial
infection of the blood).
disease-causing microorganisms including fungi
in body fluids and wounds.
stool examination for ova and parasites
disease-causing parasites such as amoebae,
pinworms, hookworms, etc.
disease-causing organisms of the lower
respiratory tract; evaluation of pneumonia and
culture and sensitivity (of many body fluids or
sites and wounds)
and identifies disease-causing organisms; tests
for effective antibiotic therapy.
bank: This area is responsible for the
selection and preparation of appropriate,
compatible blood components (red blood cells,
platelets, and plasma) that are safe for
transfusion into patients. Blood products are
also tested to be sure they are free from
infectious diseases such as HIV and hepatitis
viruses B and C. This laboratory also evaluates
transfusion reactions by diagnosing their cause,
determining whether or not it is safe to proceed
with a transfusion, and selecting further
components that are safe for transfusing. The
blood bank may have a donor service to draw
units of blood for general use and for
autologous transfusions (for elective-surgery
patients who wish to donate their own blood
before surgery so that it will be available to
them if they need it).
Shows/What It's Used For
and Rh type, or type and cross match
blood group (A, B, AB, or O) and Rh type
(positive or negative) to ensure compatibility
of transfused blood between donor and recipient.
that blood is safe for transfusion.
antibodies on surface of red blood cells in
autoimmune hemolytic anemias, transfusion
reactions, and erythroblastosis fetalis (newborn
When you go for a
routine checkup or for evaluation of a specific symptom,
your doctor will often order a set list of tests called
a laboratory, chemical, or biochemical profile. This
standard series of tests is done on a single blood or
urine specimen. The reasoning behind this is that
screening tests occasionally lead to an unsuspected
finding, and with modern automated instruments, as many
as 30 common tests can often be done for the same cost
as four or five tests ordered separately. Although
laboratories differ, the most common component tests are
among those described in the laboratory sections of this
a typical group of tests, see table 4.2).
The value of
laboratory profiles is somewhat controversial, however,
since such broad screening tests rarely lead to the
diagnosis of unsuspected illnesses. Moreover, they can
result in abnormal findings in the absence of disease,
possibly leading to unnecessary workups. The trend among
doctors now is to be very selective and to order tests
based only on specific patient complaints. In fact,
since March 1996, Medicare policy has been to cover only
specific blood tests considered medically necessary
based on symptoms, family history, or risk profiles for
a specific disease. Although many private health
insurers continue to pay for laboratory profiles, such
organizations as the American College of Physicians (ACP)
recommend that they not be done on healthy patients
without specific risk factors.
A more effective
variation of the general profile concept is an organ
profile. This is ordered when a patient has symptoms
that suggest a disease of a specific organ or organ
system. Common examples would be a liver profile, a
lipid profile, or a thyroid profile. In these instances,
the tests comprising the profile, when considered in the
context of one another, provide a much more complete
picture of the condition of an organ or organ system
than any single test by itself. In a sense, a complete
blood count (CBC), which includes a hematocrit, red and
white blood cell counts, a hemoglobin measurement, and a
white blood cell differential, can be considered a
hematology profile. Similarly, profiles can be
customized according to a clinical problem.
computerization have increased dramatically in clinical
pathology laboratories, allowing lab staff to keep up
with the high volume and array of tests now available.
Conversion from manual to automated procedures means
that tests can be done rapidly, in large batches, with
very small or "micro" amounts of specimens,
and by fewer staff. The computer-calculated results are
highly accurate because the potential for human error
that can occur with each step of multiple-step processes
has been eliminated.
chemistry tests are routinely processed on large
instruments called multichannel analyzers, by only one
or two technicians, generating up to 20 different test
results per minute on a single patient's blood specimen.
Various instruments allow for about 80% of tests in
clinical chemistry to be highly automated. The remaining
20% of technically complex and highly specialized tests
that are less automated or performed manually can
consume more than half of laboratory staff time.
not only enhanced speed and accuracy, but has also
allowed computerized reporting of test data. Often
different analyzers from different laboratories can be
linked, enabling laboratories to produce a single report
of a patient's tests from all laboratories. For hospital
patients these reports can be generated daily, or even
more frequently, and can show data from previous days
for purposes of comparison or creating comprehensive
records. Results from the laboratory can even be
displayed directly on terminals in patient care units.
laboratories use all of the technologies described below
in analyzing test data.
This group of laboratory techniques is used to identify
such diverse substances as infectious diseases,
hormones, vitamins, drugs, cardiac enzymes, and antigens
(proteins) associated with cancer. These techniques are
exquisitely sensitive and, in some instances, capable of
measuring less than a billionth of a gram of a
technology is based on the antigen-antibody response,
that is, the ability of the body to develop antibodies
(proteins made by the immune system) to protect against
antigens (proteins on the surface of invaders such as
bacteria, viruses, or allergens). Since there is a
specific antibody for each specific antigen, it is
possible to test for one using the other. That is, if
one is placed in close proximity to the other, such as
happens when a small amount of a known antigen is placed
in a test tube that is then filled with blood that
contains an antibody for it, they will bind together and
can be identified by using a special marker.
testing relies on tagging antibodies or antigens with
different markers such as radioisotopes, enzymes, or
fluorescent chemicals, in order to make the substance
tested for visible. For example, in radioimmunoassay (RIA),
the antigens are tagged with a radioisotope; in some
nonisotopic immunoassay, with a special dye that shows
up as glowing particles under a fluorescent microscope.
In enzyme-linked immunosorbent assay (ELISA)an
increasingly popular technique used in such diverse
circumstances as testing for allergies and antibodies to
the HIV virusthe antibodies are tagged with enzymes.
There are many
types of immunoassays. In addition to RIA and ELISA, the
most common ones include enzyme-multiplied immunoassay
technique (EMIT), fluorescence polarization immunoassay,
and enzyme immunoassay.
This technique measures the intensity of color formed
when the substance being tested for reacts with added
chemical reagents. It is the basis for most automated
tests performed on multichannel analyzers. Applications
vary from highly automated tests to simple procedures
for blood sugar testing that can be performed at home.
This technique is based on the differences in movement
of electrically charged particles under the influence of
an electrical current. The size, shape, and electrical
charge of the particles will affect the direction,
distance, and rate of movement and can be used to
distinguish between two substances, such as different
proteins. There are several variations on this
technique, including the following:
electrophoresis. Separates serum proteins, which is
useful in diagnosing and monitoring multiple myeloma and
related disorders, evaluating and monitoring chronic
inflammatory conditions, and evaluating and managing
kidney disease, liver disease, and nutritional status.
Proteins are separated in an electrical field and
further identified by reaction with specific antibodies.
This is most often used for diagnosing and classifying
multiple myeloma, and also for diagnosing some
electrophoresis. Separates the various types of
hemoglobin (red pigment in red blood cells). Used in
diagnosing sickle-cell anemia, thalassemia, and related
congenital blood disorders.
A technique for separating substances on the basis of
their molecular size, or physical or chemical
properties, which is used to measure drugs, some
proteins, and hormones, among other substances.
Variations of this technique are based on the medium in
which the chromatography is performed. These include
high-performance liquid chromatography (HPLC),
thin-layer chromatography (TLC), and gas-liquid
Spectrometry. In combination with
chromatographic techniques described above, this
technology makes possible very specific identification
and measurement of substances on the basis of their
physical structure. It is widely used in screening for
illicit drug use, to provide definitive confirmation
when an initial immunoassay screen is positive.
Absorption and Flame Emission Spectrometry. When
a solution is converted to the gaseous state in a hot
flame, dissolved metals, depending on the conditions,
will either emit or absorb light at a wavelength that is
characteristic of that element. A type of
spectrophotometer measures the amount of light emitted
or absorbed. Many trace metals can be measured by these
techniques, including the amount of lead in the blood, a
measurement of great importance in pediatrics.
Ion Electrode. The potential, or voltage, of
specially designed electrodes is altered when exposed to
certain elements (ions) in the blood that are critical
for normal metabolic functioning. The current generated
by the electrode is a measure of the amount of the
substance present. This technology is used in many
multichannel automated analyzers, as well as smaller
instruments that are devoted to urgent laboratory
services. Critically important tests that employ
specific ion electrodes include those used to measure
levels of the electrolytes sodium, potassium,
bicarbonate, and calcium.
Blood Cell Counters. Used in the hematology
laboratory for automated counting and typing of blood
cells (CBC). These are commonly used for evaluating and
monitoring anemia, infection, bleeding, leukemias, and
cancer chemotherapy, for screening before surgery, and
for general health screening.
Cytometry and Molecular Diagnostics. Two
relatively new techniques that are used in both
anatomical pathology and clinical pathology
laboratories. (For more information, see the discussion
of new diagnostic technologies below.)
Profile Tests and What They Most Often Mean
liver or kidney failure; gastrointestinal
disease; biliary tract disease; some bone
disease; hemolytic anemia.
failure; dehydration, blood in gastrointestinal
gland hyperfunction; thyroid gland hyperfunction,
certain cancers; various bone diseases, Vitamin
failure, parathyroid gland hypofunction;
malnutrition; Gl malabsorption; Vitamin D
imbalance resulting from gastrointestinal,
adrenal, and renal disease.
loss; some kidney disease; adrenal gland
hyperfunction, acid-base imbalance.
imbalance from a variety of causes including
respiratory failure and vomiting.
imbalance from a variety of causes including
kidney disease, diabetic acidosis, and diarrhea.
mellitus; adrenal gland hyperfunction;
intravenous glucose fluids.
insulin, liver failure, adrenal gland
failure; parathyroid gland hypofunction.
hyperfunction; malnutrition; Gl malabsorption;
vitamin D deficiency.
failure; adrenal gland hypofunction; acid-base
therapy; diarrhea; vomiting; adrenal gland
hyperfunction; acid-base imbalance.
disease or damage; heart injury; muscle injury.
adrenal gland hyperfunction; some kidney
disease; diabetes insipidus.
failure; adrenal gland hypofunction;
gastrointestinal loss; diuretics; overhydration;
kidney, liver, and heart failure.
myeloma; chronic infection or inflammation.
liver or kidney failure; gastrointestinal
kidney failure; some blood malignancies; tissue
uncommon congenital diseases.
Test results from
clinical pathology laboratories are usually represented
as numbers, or occasionally, as positive or negative,
meaning that the substance or disorder being tested for
is or is not present. The numerical values are usually
expressed as the amount of a substance present in a
given quantity of body fluid. For example, the amount of
phosphorus is given in milligrams per deciliter (100 mL)
of blood (mg/dL).
numerical values, there is a range of what is considered
normal. These so-called reference ranges can vary with
such factors as sex and age. For example, the normal
concentration of serum alkaline phosphatase, a bone
enzyme, may be up to three times greater in growing
children than in adults. Reference ranges can also vary
according to the population being considered. For
example, hospitalized patients may have a range of
results that differs from that of healthy outpatients.
reference ranges are often relatively wide, any given
individual may have a narrower range of what is normal.
Thus, you may have a test result that falls within the
normal range for the general population and still have a
disorder. There is no way to perfectly estimate ranges
because, in general, only 95% of a population is
statistically described by a reference range. Thus, it
is also possible for the remaining 5% to fall outside a
reference range and still be healthy.
reference ranges can vary from laboratory to laboratory,
depending on the technology used to analyze test
results. For these reasons, reference ranges are not
given in this book.
and Sensitivity. The most reliable tests are
those that are both sensitive and specific, and thereby
minimize the incidence of false-positive and
false-negative results. "Sensitivity" refers
to the ability of a test to correctly identify
individuals who have a given disease or disorder.
"Specificity" refers to the ability of a test
to correctly exclude individuals who do not have
a given disease or disorder.
result indicates a disease in a patient who in fact does
not have the disease, and these must be minimized to
achieve high specificity. A false-negative result
indicates that there is no disease in a patient who does
have the disease, and these must be minimized to achieve
An ideal test
that is 100% sensitive and specific would detect
everyone with a given disease but no one without it.
Few, if any, tests achieve this, although many come
close. Yet even with the most specific and sensitive
tests, misapplication can cause difficulties. For
example, even a highly specific test used in a segment
of the population that is known to have very few cases
of a disease (defined as a low prevalence) can have a
high likelihood of yielding a false-positive result in
healthy individuals. An illustration of this would be
the PSA test for prostate cancer performed on males less
than 40 years old: many of the positive results will be
Tests are only a
piece of the diagnostic process; your doctor's judgment
is critical. Presented with clinical laboratory results,
your doctor must interpret the information in the
context of your history and physical examination, as
well as other diagnostic tests. If your test results are
negative but your doctor suspects (based on your
symptoms and complaints and the physical exam) that you
have a specific disease, he or she may elect not to rule
it out but to repeat the test or order a different test
for the same disorder. The results of several tests
taken together can provide a better evaluation of a
disorder than a single test alone.
you have an abnormal test result but no signs or
symptoms of a particular condition, your doctor may
suspect a false-positive result, which might be caused
by elements in your diet or a medication you're taking,
or simply because you're one of a small percentage of
the population that falls outside the reference range.
Finally, if your tests were ordered for monitoring
purposes, your doctor may consider changes in results to
be more important than any absolute value.
DID YOU KNOW?
tubes are color-coded by stopper, indicating the type of
anticoagulant or preservative they contain. For example:
tops" contain no anticoagulant; thus they allow
the blood to clot so that serum can be drawn off.
tops" have an anticoagulant to prevent the
blood from clotting.
tops" contain a preservative that prevents the
breakdown of glucose, a blood sugar.
interfere with some tests. Always advise your doctor of
any drugs you are taking. Sometimes a medication must be
stopped before a test is performed, but usually
notification is all that is necessary.
DID YOU KNOW?
of immunoassay technology was considered so significant
that its inventors were awarded a Nobel Prize. This
technology has become so ubiquitous that it is done
manually in specialized laboratories, on automated
instruments, in doctors' offices, and in some instances,
even at home, as is the case with over-the-counter
- Abnormal test
results do not always indicate disease.
- Findings can
be affected by factors ranging from medications to
diet to athletic conditioning.
- Some tests are
more likely than others to produce false-positive
results. There is also the possibility of laboratory
- Diagnosis or
treatment should not be finalized on the basis of a
single test, especially if you have no symptoms.