COMMON LABORATORY TESTS - a patient's guide
- Laboratory test (lab test) results are subject to certain errors and in most cases require 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 results can 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 because different 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 because a 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 to them and good laboratories control and minimise these errors on a regular basis.
Laboratory tests in common use:
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 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 name suggests, 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 B12 deficiency 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 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) will be 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 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
- Streptococcal bacteria
- 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)
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 incorrectly interprets 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.
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
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.
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 streaked onto agar plates and incubated at body temperature.
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.
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,
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 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.