The Complete Blood Count (CBC) test does not directly check the filtering or waste-removal function of the kidneys. The CBC and kidney function tests are fundamentally different blood panels that measure distinct aspects of the body’s health. While healthcare providers often order both tests simultaneously, the CBC focuses on the cellular components of the blood, while a separate metabolic panel analyzes the chemical balance and waste products managed by the kidneys.
What the Complete Blood Count Measures
The Complete Blood Count is a test primarily focused on hematology, which is the study of blood and blood-forming tissues. It provides a numerical assessment of the three main types of cells circulating in the bloodstream. The three main cellular lines measured are red blood cells, white blood cells, and platelets.
Red blood cells (RBCs) are analyzed for their quantity, size, and oxygen-carrying capacity, determined by hemoglobin and hematocrit levels. Hemoglobin binds to oxygen in the lungs and transports it to tissues throughout the body. The platelet count measures the cell fragments necessary for forming blood clots and stopping bleeding.
The white blood cell (WBC) count is a measure of the immune system’s status. It reflects the number of infection-fighting cells, which can be further broken down into a differential count that identifies the proportions of five specific types, such as neutrophils and lymphocytes. Elevated or decreased numbers in any of these cellular components can point toward conditions like infection, inflammation, or anemia, but they do not measure the kidney’s ability to filter waste.
The Tests That Assess Kidney Function
To assess how well the kidneys filter blood and maintain chemical balance, a different set of laboratory measurements is required. These tests are typically included in a Basic Metabolic Panel (BMP) or a Comprehensive Metabolic Panel (CMP). Unlike the CBC, these panels measure the concentration of specific waste products and electrolytes in the plasma.
The primary measurement for kidney function is the estimated Glomerular Filtration Rate (eGFR), which is a calculated value. The eGFR estimates how many milliliters of blood the kidneys clean per minute, providing a clear picture of their filtration efficiency. This rate is calculated using a formula that incorporates the blood level of creatinine, along with the patient’s age and sex.
Creatinine is a waste product generated by the normal breakdown of muscle tissue, which healthy kidneys continuously remove from the blood. When kidney function declines, creatinine accumulates in the bloodstream, leading to higher-than-normal serum levels. Another substance measured is Blood Urea Nitrogen (BUN), a waste product of protein metabolism produced in the liver.
Like creatinine, BUN levels rise when the kidneys cannot effectively excrete this nitrogenous waste. The CMP also measures electrolytes, such as potassium and sodium, which the kidneys regulate to maintain fluid and chemical balance. Changes in these levels indicate impaired kidney regulation.
How Kidney Health Affects CBC Results
Although the CBC does not directly assess kidney function, poor kidney health has a measurable effect on the CBC results, establishing an important indirect link. This connection is primarily seen in red blood cell parameters, specifically hemoglobin and hematocrit, which demonstrate anemia. Anemia is a common complication of Chronic Kidney Disease (CKD) due to the kidney’s role as an endocrine organ.
The kidneys produce a hormone called erythropoietin (EPO), which signals the bone marrow to manufacture new red blood cells. When kidney tissue is damaged over time, as happens in CKD, the production of EPO is reduced. A sustained lack of EPO leads to insufficient stimulation of the bone marrow, resulting in a decreased number of circulating red blood cells.
This reduction in red blood cells is reflected in the CBC as anemia, evidenced by low hemoglobin and hematocrit values. A low hemoglobin level on a CBC for a patient with known kidney problems often confirms the disease’s systemic impact on blood cell production. While a CBC cannot diagnose the start of kidney failure, this specific type of anemia provides a clear clue about the severity of the underlying kidney disease.