Top 10 laboratory tests: blood will tell - includes related information - part 1

FDA Consumer, July-August, 1989 by Dixie Farley

Significance of results: Abnormal levels may reflect problems with any of the complex body systems needed to process glucose. Usually, diabetes mellitus is at the root of excess blood sugar. But there are other causes, such as tumors of the pancreas, severe stress, thyroid overactivity, and use of diuretics. Low blood sugar levels, on the other hand, seldom indicate illness. Blood sugar normally drops after exercise or after a meal is digested. Low levels are seen in diabetics who overdose with insulin and in infants, because they process food quickly.

Creatinine and Kidney Function

Measurement: The level of creatinine in serum.

Normal range: 0.7 to 1.5 mg/dL.

Use: This and the blood urea nitrogen test (see below) are commonly used together to test kidney function, though the creatinine test, considered more sensitive, is often used alone.

Comments: Creatinine is a waste product of normal protein breakdown in muscle tissue. Ordinarily, the kidneys filter creatinine from the blood at a steady rate and dispose of it in the urine. Testing for the substance in serum indicates whether the rate is normal and the kidneys are working properly.

Significance of results: A high level mainly results from kidney disease. It may, however, merely reflect heavy exercise or a diet high in red meat. Low serum creatinine levels are seen in patients with muscle wasting diseases such as muscular dystrophy.

Blood Urea Nitrogen (BUN) and Kidney Function

Measurement: The level of urea nitrogen in serum.

Normal range: 4 to 22 mg/dL.

Use: To test kidney function (but rarely used alone for this purpose).

Comments: Like creatinine, urea nitrogen is a waste product of protein metabolism. It is produced by the liver and excreted by the kidneys. BUN and creatinine increases provide indicators (along with others) of chronic kidney failure and the need for dialysis.

Significance of results: As with creatinine, BUN is elevated mainly in kidney disease. But high levels also may accompany dehydration, long-term fever, bleeding ulcer, or congestive heart failure that has stemmed blood flow to the kidneys. Lethargy and confusion are frequent signs of increased urea nitrogen. Decreased BUN in the absence of other signs probably is insignificant.

Lactic Dehydrogenase and Heart Attack

Measurement: The amount of lactic dehydrogenase in serum.

Normal range: 60 to 220 international units per liter.

Use: Mainly to monitor patients with diagnosed or suspected heart attack.

Comments: The enzyme lactic dehydrogenase (LDH) plays an important role in making carbohydrates available to the body. LDH is found chiefly in the skeletal muscles, liver, heart, pancreas, spleen, and brain and, at lower concentrations, in lymph nodes, lungs, and thyroid and adrenal glands. When injured, those tissues release LDH into the bloodstream in amounts proportional to the extent of the damage.

Significance of results: LDH blood levels rise 12 to 24 hours after a heart attack and stay high as long as inflammation persists. For this reason, physicians use this test to help determine whether a heart attack has occurred and may, in fact, test patients daily when heart attack is suspected. Increased LDH may also help to assess severity of liver disease. High levels can mean kidney disorders, infectious mononucleosis, or pernicious anemia, but further tests may be needed to specify which tissue is involved. Low LDH has no health significance.