RDW-CV (Red Cell Distribution Width – Coefficient of Variation) measures how much your red blood cells vary in size, expressed as a percentage. The normal range is approximately 11.5–14.5%. A high RDW-CV indicates anisocytosis — greater variation in cell size — most commonly from iron, B12, or folate deficiency. A low RDW-CV means cells are unusually uniform and is usually not clinically significant. RDW-CV is always interpreted alongside MCV and haemoglobin, not in isolation.

If your result is in fL (femtoliters) rather than %, it is RDW-SD — see our dedicated RDW-SD page.

Red Cell Distribution Width-Standard Deviation (RDW-SD) is a blood test parameter that measures the variability in the size of your red blood cells. It's one of the ways scientists assess the volume and size of red blood cells within a sample. A standard complete blood count (CBC) will often include RDW as it measures many different parts of your blood, including red blood cells. RDW can be reported in different ways, depending on the types of hematology analyzer instruments used, commonly as RDW-CV (Coefficient of Variation) or RDW-SD (Standard Deviation).

The RDW-SD specifically refers to the standard deviation of red blood cell volume, indicating how much the size of red blood cells varies around the average size. Normal RDW-SD levels are typically between 39-46 fL for adults. When the RDW-SD level is higher than normal, it may suggest that there is a significant variation in the size of red blood cells, which can be a symptom of various health conditions, such as anemia, liver disease, or other medical issues.

The red blood cell (RBC) count measures how many red cells — also called erythrocytes — are present in a given volume of blood. Red cells carry oxygen from the lungs to the tissues and return carbon dioxide to the lungs, using the iron-containing protein hemoglobin. The erythrocyte count is a core part of the complete blood count (CBC) and is interpreted alongside hemoglobin, hematocrit, and the red cell indices (MCV, MCH, MCHC, RDW). A low count is called anemia and most often reflects iron, B12, or folate deficiency, blood loss, chronic disease, or reduced production. A high count is called erythrocytosis (or polycythemia) and most often reflects dehydration, low oxygen levels, smoking, or — less commonly — a bone marrow condition. Because the number shifts with hydration, sex, altitude, and pregnancy, the red cell count is rarely meaningful on its own — its value comes from the pattern it forms with the rest of the CBC.

Red blood cells can enter the urine from the vagina in menstruation or from the trauma of bladder catheterization (a procedure used to diagnose and treat cardiovascular conditions).

Rfx to HBc IgM (Reflex to Hepatitis B Core IgM Antibody)

What this test measures
The Rfx to HBc IgM test looks for IgM antibodies to the Hepatitis B core antigen, which are produced by your immune system during the early phase of a Hepatitis B infection.
“Rfx” means reflex testing — this test is automatically performed only if an initial Hepatitis B screening test (usually HBsAg) comes back abnormal or positive. It helps confirm whether an infection is recent or acute, rather than past or chronic.

Renin is an enzyme produced by the kidneys when blood volume, hydration, or salt levels are low. Its main function is to help regulate blood pressure and maintain fluid balance.

Renin activates a chain of reactions known as the renin–angiotensin–aldosterone system (RAAS). It converts a protein called angiotensinogen into angiotensin I, which is then transformed into angiotensin II. Angiotensin II constricts blood vessels, raises blood pressure, and signals the adrenal glands to release aldosterone. In turn, aldosterone prompts the kidneys to retain sodium and water, helping restore normal blood pressure and fluid levels.

If this system becomes overactive, it can lead to high blood pressure (hypertension) and may contribute to kidney or cardiovascular disease.

The reptilase clotting time measures the rate of fibrin clot formation after the addition of reptilase, a proteolytic enzyme derived from the venom of Bothrops atrox, to citrated plasma.

Reptilase is a thrombin-like enzyme. 

Unlike thrombin, which cleaves fibrinogen to produce fibrinopeptides A and B, reptilase cleaves the fibrinogen molecule to release only fibrinopeptide A.

This section explores the presence and abundance of a panel of antibiotic resistance genes in the gut microbiome, and compares that to healthy population. The resistome is the sum of antimicrobial resistance genes in your gut. High number of resistance functions in your gut will make pathogens less likely to be sensitive to antibiotics and may be an indication that virulent strains are flourishing.

Reticulocyte Hemoglobin Equivalent (Retic Hgb Equivalent) is an invaluable marker in hematology, gaining prominence for its role in assessing iron status and erythropoiesis in clinical settings. This marker measures the hemoglobin content in reticulocytes, which are immature red blood cells recently released from the bone marrow. Retic Hgb Equivalent provides critical insights into the iron available for new red blood cell production, making it a vital tool for diagnosing and managing iron deficiency anemia and other disorders of red blood cell production.

Reticulocytes are newly produced, relatively immature red blood cells (RBCs). A reticulocyte test determines the number and/or percentage of reticulocytes in the blood and is a reflection of recent bone marrow function or activity.

Reticulocyte hemoglobin (Ret-He) is a crucial indicator in hematology, measuring hemoglobin in immature red blood cells, or reticulocytes. It's pivotal for diagnosing and managing anemia, especially iron deficiency anemia. Ret-He offers an early sign of iron deficiency, aiding in prompt intervention. This measure is more dynamic than traditional markers like serum ferritin, reflecting real-time iron availability for red blood cell production. Its inclusion in modern blood count analyzers underscores its importance in accurate diagnosis and patient care in iron-related disorders.

Reticulocytes are red blood cells that are still developing. They are also known as immature red blood cells. Reticulocytes are made in the bone marrow and sent into the bloodstream. About two days after they form, they develop into mature red blood cells. These red blood cells move oxygen from your lungs to every cell in your body.

RBP is a sensitive marker of undernutrition and the monitoring of its serum concentration allows the monitoring of nutritional status.

Retinol-binding protein (RBP) is an unglycosylated protein synthesised by the liver. The role of RBP is to transport retinol (vitamin A) from the liver to the target tissues (retina, skin, etc.) in the bloodstream. This protein is then metabolized, filtered and reabsorbed in the kidney.