A complete blood count (CBC) is a common diagnostic tool for overall health. While it doesn’t directly measure kidney function, a CBC can offer indirect clues suggesting an underlying kidney problem, prompting further investigation.
Understanding the Complete Blood Count (CBC)
A complete blood count (CBC) measures various blood components, providing a comprehensive overview of blood health. This test evaluates red blood cells, which transport oxygen; white blood cells, which fight infections; and platelets, essential for clotting. A CBC also measures hemoglobin, the oxygen-carrying protein, and hematocrit, indicating the proportion of blood volume occupied by red blood cells.
Indirect Clues from a CBC for Kidney Health
While a CBC does not directly assess kidney function, certain changes in its results can provide indirect indications of kidney issues. One notable clue is the presence of anemia, a condition characterized by a reduced number of red blood cells or low hemoglobin levels.
Kidneys produce a hormone called erythropoietin (EPO), which signals the bone marrow to produce red blood cells. When kidney function declines, insufficient EPO production can lead to anemia.
Anemia is a common complication of chronic kidney disease (CKD) and often becomes more pronounced as kidney damage progresses. Besides reduced EPO, other factors like iron deficiency and a shorter lifespan of red blood cells in individuals with kidney disease can also contribute to anemia. Therefore, an unexplained or persistent anemia detected on a CBC might warrant further evaluation of kidney health.
Specific Tests for Kidney Function
Directly assessing kidney function requires specific laboratory tests that measure the kidneys’ ability to filter waste products from the blood and produce urine.
Serum creatinine is a widely used marker; creatinine is a waste product from muscle metabolism that healthy kidneys filter out of the blood. An elevated serum creatinine level often indicates that the kidneys are not functioning efficiently.
Another important indicator is blood urea nitrogen (BUN), which measures urea nitrogen, a waste product formed when the liver breaks down protein. Like creatinine, BUN is filtered by the kidneys, and high levels can suggest impaired kidney function. While both creatinine and BUN are valuable, their levels can be influenced by factors such as muscle mass, diet, and hydration.
The glomerular filtration rate (GFR) is considered the best overall measure of kidney function. GFR estimates how much blood the kidneys filter per minute and is typically calculated using serum creatinine levels, along with age, sex, and sometimes other factors. A lower GFR indicates reduced kidney function.
Urinalysis is another fundamental test for kidney health, involving the examination of a urine sample. This test can detect substances that should not be present in significant amounts, such as protein (especially albumin) or red blood cells, which can signal kidney damage. The presence of certain types of casts or an abnormal appearance of the urine can also provide clues about kidney conditions.