common laboratory tests for families

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COMMON LABORATORY TESTS - a patient's guide

Dr Logan Mclennan- Family Doctor

Overview:

Laboratory test (lab test) results are subject to certain errors and in most casesrequire interpretation depending on the patient's symptoms and signs.

An abnormal lab test will normally require repeating to confirm the result Doctors have an old saying. "Treat the patient not the lab test" meaning that an

isolated abnormal lab result may not mean an abnormality in the patient.

Different laboratories can use different methods for the same test so that resultscan not always be directly compared between laboratories. A laboratory will have its

own set of normal values.

A normal range of values for each lab test is not provided in this article becausedifferent measurement systems are in use in different parts of the world and

because each laboratory will have its own reference range of values.

Medical laboratory tests are often grouped when they are reported. This is becausea group of tests may give more information about a disease or body organ than one

single test. This article describes groups of tests where appropriate because this is

the way doctors usually look at test results.

How do laboratories carry out testing?

The laboratory needs to receive a sample from a person in good condition for testing. To

minimise errors, the sample has to be collected properly and transported to the lab in the

correct transport containers. Samples can include skin scrapings, blood, swabs from skin,

eyes, ears and bodily orifices, urine and faeces, hair and small pieces of tissue or larger

lesions cut out of the body.

Many lab tests are carried out on automated testing machines or with special kits which

contain testing chemicals of controlled quality so that the tests are reproducible and

reliable. Other tests require considerable manual handling. All tests have errors built in tothem and good laboratories control and minimise these errors on a regular basis.

Laboratory tests in common use:

Blood tests:

Human blood contains thousands of different chemicals and molecules. If blood in a test

tube is allowed to stand it separates out into blood cells and the support fluid called plasma.

Some tests are done on the blood cells and others just on the plasma. Blood also has a

unique ability to clot and will normally do so when taken out of the body into a test tube

unless an anti-clotting agent is added. If clotted blood is tested the fluid left separate from


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the clotted cells is called serum. Separating blood cells from the support fluids can be

speeded up by spinning the test tube of blood in a centrifuge at high speed.

The Complete Blood Count: (CBC)

This is a common group blood test, which assesses the cells in blood. It is not, as the namesuggests, a test which completely tests everything in blood. It looks mainly at the three main

types of cells in blood - red blood cells, white blood cells and platelets.

The red blood cells contain a molecule called haemoglobin. This carries oxygen from the

lungs to the tissues and can be low if a person has been losing blood or if their iron intake is

low. Iron is needed to make haemoglobin. The complete blood count also reports the

number of red blood cells, their size and shape and the concentration of haemoglobin within

each cell. The doctor first looks at the report to see if the person is anaemic i.e. low in

haemoglobin and then looks for clues as to the cause. An anaemia due to recent blood loss

may show a certain picture, that due to iron deficiency another and that due to vitamin B12deficiency yet another.

The white blood cells are a major part of the body's defences against invading organisms.

The doctor will look at the total white cell count and then the percentages of different types

of white cells. For example if the patient has pneumonia due to bacteria the neutrophil

white count may be very high because the immune system produces this type of white cell

to fight bacteria. Or if the patient has intestinal parasites he or she may have a high

eosinophil white cell count. Leukemia, which is a cancer of bone marrow, can be diagnosed

by seeing abnormal white cells in the blood.

Platelets are part of the blood clotting system, which is essential to prevent excessive blood

loss from a cut.

Serum Iron Tests

This is a group that tests whether iron levels are too low which might lead to a low

haemoglobin (iron deficiency anaemia) and whether there is an iron storage disorder. In this

latter situation the body tries to store too much iron which can be toxic to tissues (e.g. a

disease called haemochromatosis). Iron travels around in the blood attached to special

transport proteins. This group also assesses the transport proteins because a low

haemoglobin count can occur where there are plenty of iron stores but no way to transport

the iron.

Serum Vitamin Assays

The commonest vitamins assayed in blood are Vitamin B12 and Folic Acid because low levels

of these can lead to anaemia particularly in the elderly (e.g. a disease called pernicious

anemia).

Blood Clotting Tests:

Sometimes it is important to test the ability of the blood to clot properly. As well as the

platelet count described above, a test called the INR (International Normalised Ratio) is the


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commonly performed of these tests. Patients, who need to be on anticoagulant medications

to reduce the blood's clotting ability, need to have an INR performed regularly to ensure

that the ideal levels are achieved. Other tests in this group include the prothrombin time,

the bleeding time and tests of other blood clotting factors

Liver Function Tests:

This group of tests indirectly assesses the health of liver cells by measuring enzymes arising

from the cells and also assesses substances produced by the liver and used elsewhere in the

body. The liver acts a bit like a factory. If the liver enzymes are raised it can suggest damage

to liver cells such as occurs with chronic alcoholism or with certain viral infections such as

viral hepatitis. Other tests in this group can suggest problems occurring outside the liver

such as a gallstone blocking the duct that runs from the liver to the small intestine which

conducts bile. Bile helps the intestine dissolve fat in the diet so that it can be used as fuel.

Kidney Function Tests:

The two commonest blood tests of how well the kidneys are working, are the urea test (or

blood urea nitrogen = BUN test as it is known in some countries) and the creatinine test.

These two substances are produced by metabolism in the body and the kidney clears them

out of the blood into urine. High levels in the blood therefore suggest that the kidney may

not be working properly.

Other common tests under the umbrella of kidney function tests, include:

Uric Acid - a breakdown product of metabolism which can cause gout if the blood level is

raised.

Electrolytes - the serum levels of potassium and sodium are often measured

The calcium and phosphate levels are part of the kidney tests because kidney malfunction

can affect bone.

Thyroid Function Tests:

This group of tests assesses the thyroid gland which regulates metabolism in the body. The

whole group will be ordered if there are signs of low or high thyroid hormone output on

physical examination. Sometimes just one test - the TSH (thyroid stimulating hormone) willbe ordered to rule out a thyroid problem when symptoms might be difficult to explain.

Sexually Transmitted Disease (STD) Blood Tests

VDRL/TPHA - these are screening tests for syphilis. If positive usually further confirmatory

tests are carried out.

HIV - this is a screening test for the presence of Human Immunodeficiency Virus antibody.

Being an antibody it is measuring the body's reaction to the presence of the antigen.

(Antigen refers to any invader into the body be it virus or other agent). The antigen in this

case is the Human Immunodeficiency Virus that can lead eventually to AIDS. Antibodies can


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take time for the body to produce so they may not be measurable early in an infection. The

period for antibodies to develop may be 3 to 6 months (called the "window" period). This is

why a repeat test may be needed some time after potential exposure.

Herpes antibodies - can be measured to assess past or present herpes simplex infection.

Hepatitis antibody and antigen testing can be included in the STD tests as well as with Liver

Function Tests. The various types of hepatitis can be contracted by other means as well as

sexual transmission however.

Other blood tests for infection:

Antibodies and sometimes antigens, can be measured for other infecting agents. The

commonly performed tests include:

Epstein Barr virus Toxoplasmosis Cytomegalovirus Streptococcal bacteria Leptospirosis Rubella virus

It requires caution interpreting these test results because the antibody tests may often

indicate past infection - not current infection.

Blood Culture Test - this is usually carried out as an urgent test in cases of serious infection

where the doctor suspects that a bacteria is multiplying within the bloodstream

Blood Tests Commonly Ordered Singly

Serum Amylase - this is commonly ordered in cases of abdominal pain to see if the pancreas

gland is inflamed or its duct obstructed.

Follicle stimulating hormone (FSH) - sustained high levels of FSH in a woman can indicate

that the menopause is approaching or has happened.

Progesterone - this is done to assess the state of the ovaries.

Beta Human Chorionic Gonadotrophin (HCG) - this is the hormone produced in early

pregnancy and is the basis of the urine pregnancy tests. However sometimes the blood

levels are measured if there is concern about an ectopic pregnancy or a molar pregnancy.

Diabetes Blood Tests

Blood sugar (plasma glucose)


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Plasma glucose can be tested as a fasting specimen (FPG) or casually (CPG). Tests taken on

venous blood drawn out of an arm vein are more accurate than finger pick glucose tests.

(Capillary blood glucose). Sometimes plasma glucose tests are done as a "run" i.e. 3 tests

over several hours. This is commonly known as a blood sugar series and is usually done to

test control in established diabetes. Plasma glucose can also be measured at defined times

after the patient drinks a measured amount of glucose in water. This is done either to

establish the diagnosis of diabetes (when it is known as the glucose tolerance test - GTT) or

as a screen for pregnancy diabetes (when it is known as the glucose challenge test).

Glycated haemoglobin often abbreviated to HbA1c. This is a test of average diabetes control

back over the preceding six weeks. It is now more often used for this purpose than the

Fructosamine test which is rather similar but less accurate.

Autoimmune Disease Tests

Autoimmune diseases are a group of diseases where the body's immune system incorrectlyinterprets certain of its own tissues as a foreign invader and produces an immune response

to attack that tissue. The best known autoimmune disease is rheumatoid arthritis where the

immune system attacks the slippery lubricated lining inside certain joints. The blood tests try

to measure specific antibodies produced by the body against specific tissues. Such tests

include:

Rheumatoid factor Lupus anticoagulant test Antinuclear antibody test

The ESR test (erythrocyte sedimentation rate) is a test on red cells but is included here as it

is often used to monitor the response of autoimmune diseases to treatment. If raised it may

be a general pointer to an infective or inflammatory process going on in the body. It is not

diagnostic of any particular illness as such.

Drug Assays

The level of certain drugs can be measured in the serum. The commonly performed drug

assays are usually those where the drug has a fine line between being toxic and therapeutic.

The common assays include:

Serum digoxin Serum phenytoin Serum theophylline Serum Lithium Serum alcohol Serum Lipids


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The term lipids refers to certain fats that are transported within the bloodstream.

Cholesterol is one of these. Patients are often concerned by their total cholesterol level

which a laboratory will usually only report as a single test if a non-fasting blood specimen

was submitted. However doctors tend to request fasting lipid tests and read them as a group

because there is "good" cholesterol (HDL cholesterol) as well as "bad" cholesterol (LDL and

VLDL cholesterol). When making an assessment of the patient's future risk of heart disease

or stroke the doctor has to assess these levels as well as another fat reported in this group of

tests (Triglycerides). These are very small fatty molecules, which are usually raised after a

more fatty meal. They can also be raised by heavy alcohol intake and obesity. There is

controversy about how important they are in cardiovascular risk,but evidence is increasing

that they may be important.

Disease Marker Tests

Blood tests can monitor the levels of certain chemicals in the blood as an indicator of the

progress of a disease. The best known of these tests is the PSA (prostate specific antigen)

test which is monitored in prostate cancer. The AFP (alpha foeto-protein) is another - used

for monitoring treatment for liver cancer.

Ca-125, is used to monitor progress in ovarian cancer.

Urine Tests:

Some urine tests can be carried out in the doctor's office and others require the laboratory.

Common urine tests include:

Glucose - as an indicator of diabetes

Note-urine testing is not reliable to exclude diabetes, as sometimes no sugar is in the urine

of a diabetic and sometimes non diabetics may have sugar in their urine (due to a low

threshold in the kidneys).

Protein - which can be present with kidney disease or with infection/inflammation of the

kidneys or bladder or urethra.

Microalbumin - which can be an indicator of early kidney disease in diabetes.

Red blood cells - which indicate bleeding somewhere in the urinary tract perhaps from

infection or from a small tear to the tract as can occur with a kidney stone, or bleeding from

a tumour in the kidney or bladder.

This usually always requires further tests (e.g. scans,ivu,cystoscopy) to determine where the

blood has come from.

White blood cells - which indicate that the body is fighting an infection somewhere in the

urinary tract.

Bacteria tests - Bacteria can be seen under the microscope or grown from urine streakedonto agar plates and incubated at body temperature.


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Chlamydia test - which identifies the sexually transmitted disease chlamydia in urine.

Pregnancy test - which relies on detection of the hormone Beta Human Chorionic

Gonadatrophin in urine. This is produced in the early stages of a foetal placenta developing

in the uterus.

The test usually becomes positive about a week after the first missed period. If there is

doubt, it may need to be repeated again a week or so later.

A 24 hour collection of urine is sometimes ordered to check on protein levels or to do the

creatinine clearance test to assess kidney function or to measure urine output.

Tissue histology:

FNA = fine needle aspiration. A fine needle on a syringe is inserted into a lesion and negative

pressure is applied to suck up some cells for microscopy. This is usually to check for tumour

cells.

Excision Biopsy. This is where an entire lesion is removed and sliced up into thin sections for

microscopy.

Biopsy. This is where a small part of a lesion is removed for microscopy to try and make a

diagnosis,

Microbiology:

Any moist area on the body can be swabbed with a sterile cotton tipped bud, plated out on

to agar jelly plates and placed into an incubator to see if bacteria will grow from the swab.Once the bacteria have grown they can be identified under the microscope and tested to for

sensitivity to certain antibiotics. This is known as a the "culture and sensitivity test" (C & S).

Scrapings of tissue or cuttings of nails can be similarly treated for fungal lesions.

If there is a delay between taking the swab and plating it out, it has to be stored in special

transport media.

Sputum is commonly cultured for bacteria causing bronchitis, pneumonia and tuberculosis.

Faeces can be cultured for bacterial, viral and parasitic organisms.

Cytology:

Cytology involves obtaining a normal tissue fluid and examining it under the microscope for

early signs of cancer cells.

The two commonest cytology tests are the cervical smear test (pap smear) and sputum

analysis for lung cancer cells

common laboratory tests for families

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