Lab Tests Reference Library

Hemoglobin (Hgb) is the iron-containing protein in red blood cells responsible for carrying oxygen from the lungs to body tissues and returning carbon dioxide to the lungs. It is the primary indicator of anemia and is measured in every complete blood count (CBC).

The red blood cell (RBC) count measures the number of erythrocytes per microliter of blood. RBCs carry oxygen bound to hemoglobin and remove carbon dioxide. The RBC count is used together with hemoglobin and hematocrit to classify anemia and guide its investigation.

The white blood cell (WBC) count measures the total number of leukocytes in the blood, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils. WBCs are the immune system's primary defenders against infection, inflammation, and foreign substances.

Platelets (thrombocytes) are small cell fragments produced by megakaryocytes in the bone marrow. They are essential for primary hemostasis — forming the initial platelet plug at sites of vascular injury to stop bleeding. A platelet count is routinely measured in every CBC.

Hematocrit (HCT) measures the percentage of blood volume made up of red blood cells. It reflects the overall red cell mass and is used alongside hemoglobin and RBC count to assess anemia and polycythemia. Hematocrit is approximately three times the hemoglobin value in most clinical situations.

MCV is the average volume of individual red blood cells, measured in femtoliters (fL). It is the most important red cell index for classifying anemia as microcytic (small cells, low MCV), normocytic (normal cells), or macrocytic (large cells, high MCV). Each category has a distinct set of causes that guides further investigation.

MCH measures the average amount of hemoglobin contained in a single red blood cell, expressed in picograms (pg). It closely parallels MCV — when cells are larger, they usually contain more hemoglobin. MCH is used to classify anemia as hypochromic (low MCH) or normochromic.

MCHC measures the average concentration of hemoglobin in red blood cells as g/dL. Unlike MCH, it corrects for cell size, making it a true measure of hemoglobin packing density. MCHC is the most stable CBC index and is often used as an internal quality control measure for laboratory instruments.

RDW measures the variability in red blood cell size (anisocytosis). A high RDW means red cells vary greatly in size, suggesting mixed nutritional deficiencies, early iron deficiency, or active hemolysis. A normal RDW with low MCV points more toward thalassemia than iron deficiency.

Neutrophils are the most abundant white blood cells and the primary responders to bacterial infections. They engulf and destroy bacteria through phagocytosis. The absolute neutrophil count (ANC) is the key measure of the body's ability to fight bacterial and fungal infections.

Lymphocytes are white blood cells of the adaptive immune system, consisting of B cells (antibody production), T cells (cellular immunity), and NK cells (natural killer cells). They are the second most abundant WBC type in adults and the most abundant in children. Lymphocytes are the predominant cell in viral infections.

Monocytes are large white blood cells that circulate in blood and migrate into tissues to become macrophages. They play a key role in phagocytosis of bacteria, fungi, and dead cells, antigen presentation, and coordination of the immune response. Monocytosis can be a clue to chronic infections or myeloid malignancy.

Eosinophils are white blood cells that primarily fight parasitic infections and modulate allergic responses. They are elevated in allergic conditions like asthma, atopic dermatitis, and hay fever, as well as in parasitic infections and some malignancies. The absolute eosinophil count (AEC) is more clinically useful than the percentage.

Basophils are the least common white blood cells, making up less than 1% of circulating leukocytes. They are involved in immediate allergic reactions and inflammation, releasing histamine, heparin, and other mediators. Basophilia is rare but is the hallmark of certain myeloproliferative disorders.

ESR measures how quickly red blood cells settle to the bottom of a test tube over one hour. When inflammation is present, proteins like fibrinogen coat red cells, causing them to clump and settle faster. ESR is a nonspecific marker of inflammation used to screen for and monitor inflammatory conditions.

CRP is an acute-phase protein produced by the liver in response to inflammation, infection, and tissue injury. It rises within 6–12 hours of an inflammatory stimulus and falls quickly when inflammation resolves. CRP is more specific and sensitive than ESR for detecting acute inflammation.

Prothrombin time (PT) measures how long it takes blood to clot via the extrinsic coagulation pathway, which includes factors I, II, V, VII, and X. The International Normalized Ratio (INR) standardizes PT results across different laboratories. PT/INR is used to monitor warfarin therapy and assess liver synthetic function.

aPTT measures the time for blood to clot via the intrinsic coagulation pathway, which includes factors XII, XI, IX, VIII, X, V, II, and I. It is used to detect hemophilia A and B, monitor unfractionated heparin therapy, and screen for lupus anticoagulant.