ASK DR. BAUGHAN                                             February 19, 1999

COMMON LAB TESTS

A patient suggested I write about some of the most common lab tests ordered and what they mean, so here goes:

First, numbers may not lie, but they can mislead and confuse.  Every blood test is susceptible to the problems of “false positive” results, meaning the test says you have a problem but you really don’t, and “false negatives,” when the test says you are “normal” but there is a problem.  The “range of normal” for most blood tests reported with numbers (compared to tests reported positive or negative) was determined by having thousands of seemingly normal people take a test.  The top 2.5% of the numbers were called “too high” and the bottom 2.5% were called “too low.”  So for any given test, 5% of people might be healthy but have lab results “outside the normal range.”  When this mythical convention occurred to decide on this 2.5% rule has been lost in the Mists of Avalon, but its legacy survives throughout the world.  This is why any test must be weighed against the evidence of the patient’s history and present symptoms.

A BLOOD COUNT (also called a CBC, for complete blood count, although “complete” is a presumptuous and inaccurate adjective) counts the numbers of white blood cells and red blood cells and platelets (the cells that we need to make clots when we bleed).  Also measured is the size of the red blood cells and the amount of hemoglobin they carry on average.  These numbers can help determine if someone is anemic (low red blood cell total).  Higher than normal white blood cells can support the diagnosis of infection, but a person can have a infection and have a normal or even low white blood cell count.  There are 5 most common types of white blood cells.  One type, called eosinophils, might be disproportionately plentiful when there are allergies or parasitic infections.  A normal CBC gives good evidence that the bone marrow is functioning normally.

BLOOD SUGAR, or more accurately “serum glucose,” is used to check for diabetes.  More than one elevated reading is necessary to confirm that a level is consistently or frequently high, since the glucose with vary with eating and activity.  The normal range is now considered to be 60 - 110 for a “fasting glucose,” meaning the person has not eaten for 10-12 hours.  This is lower than it used to be.   Another test, called the Hemoglobin A1c, or glycosylated hemoglobin, can give a number than reflects the average serum glucose for the past 2-3 months.  Hypoglycemia (low blood sugar) remains a controversial and elusive diagnosis.  The best way to resolve any concerns about hypoglycemia is to check a level when the person is having symptoms.

THYROID TESTS - We used to frequently order several thyroid blood tests if we suspected a thyroid disease.  There were considerable false negatives until we were able to measure the Thyroid Stimulating Hormone (TSH).  This is much more sensitive and specific and has all but replaced the others.

LIVER TESTS - Because the liver produces so many chemicals, there are multiple tests to monitor liver function.  Liver cells are always dying and regenerating, which means a constant turnover of certain enzymes that we can measure.  If too many cells are dying, certain enzymes may be too high.  If the liver is severely damaged, the cells may not make enough enzymes and the levels would be low.  Monitoring a sequence of tests over several months can sometimes be needed to clarify the situation.  Other organs, like muscles, bone, and the heart, produce some of the same chemicals as the liver, so those parts of the body must be considered also.  Other types of tests can check for antibodies and other evidence of hepatitis viruses.

KIDNEY TESTS - Urine can be tested to check for glucose and other chemicals in excess, such as proteins or ketones.  Blood cells in the urine suggest certain problems that require testing.  The blood tests Blood Urea Nitrogen (BUN) and creatinine measure chemicals the kidneys should be eliminating each day.  If the body is producing too many of these, or if the kidney cannot excrete them, high levels build up.

ARTHRITIS TESTS (RA, ANA, ESR - also called the sedimentation (or sed) rate.  Positive tests support but do not make the diagnosis of rheumatoid arthritis or systemic lupus erythematosus without other symptoms and physical findings.  On the other hand, if a person has a strong history for these diseases but their test is negative, they cannot be reassured that they do not have a problem.

I have previously discussed the debate in interpreting cholesterol levels and prostate tests (PSA).  In summary, no test is 100% reliable.  Look at the patient, talk to the patient.  The famous physician, Sir William Osler, once said “talk to a patient long enough and they will tell you what is wrong with them.