A red blood cell (RBC) count is a standard blood test that measures the number of red blood cells within a person’s blood. These cells are responsible for transporting oxygen from the lungs to the body’s tissues. The oxygen is carried by a protein inside the RBCs called hemoglobin. The amount of oxygen delivered is directly related to the number of red blood cells and how well they function.
Normal RBC Ranges by Age Group
The expected range for a red blood cell count shifts throughout a person’s life, from birth through adulthood. These reference ranges, measured in million cells per microliter (cells/mcL), can vary slightly between laboratories but follow a predictable pattern of change.
Newborns have the highest RBC counts seen at any point in life. For the first day, a normal range is between 4.0 and 6.6 million cells/mcL. This number remains high for the first couple of weeks, measuring between 3.6 and 6.3 million cells/mcL. This elevated level is a remnant of their development in a lower-oxygen environment.
Following the newborn period, the RBC count declines as an infant adjusts to life outside the womb. Between two and six months of age, the count can drop to a range of 3.1 to 4.5 million cells/mcL. From six months to one year, the numbers stabilize and rise, with a range of 4.1 to 5.3 million cells/mcL. This trend continues through early childhood, with ranges of 3.9 to 5.3 million cells/mcL for children aged one to six years and 4.0 to 5.2 million cells/mcL for those six to twelve.
During adolescence and adulthood, the RBC count solidifies, and distinct differences between males and females emerge. For adult females, a normal range is 4.2 to 5.4 million cells/mcL. In contrast, adult males have a higher count, with a normal range of 4.7 to 6.1 million cells/mcL. These sex-specific ranges appear after puberty and remain consistent through most of adult life.
Physiological Reasons for Age-Related Variations
The fluctuations in red blood cell counts are driven by the body’s changing physiological needs. As an infant grows into a child, the RBC count gradually increases to support the expansion of body tissues and a higher metabolic rate. More cells are needed to deliver oxygen for rapid growth and increased physical activity. Before puberty, RBC counts are similar between boys and girls.
The divergence in RBC counts between males and females begins during puberty, primarily due to hormonal influences. The male hormone testosterone stimulates the kidneys to produce erythropoietin, a hormone that signals the bone marrow to increase red blood cell production. This results in a higher count for males. In females, the onset of menstruation introduces a regular loss of blood, which contributes to a comparatively lower average RBC count.
Factors That Influence RBC Count Results
Besides age and sex, several external and lifestyle factors can influence RBC count results.
- Altitude: People living at high altitudes, where oxygen levels are lower, often have higher RBC counts as their bodies adapt to improve oxygen transport.
- Dehydration: This can affect test results by decreasing blood plasma volume, which concentrates the red blood cells and can lead to a falsely elevated count.
- Smoking: This habit is known to increase the RBC count as a response to the reduced oxygen-carrying capacity of the blood caused by carbon monoxide.
- Medications: Certain medications can alter RBC levels, either increasing or decreasing the count depending on their mechanism of action.
Implications of Abnormal RBC Counts
An RBC count that falls outside the expected range for a person’s age and sex may indicate an underlying medical condition. A lower-than-normal RBC count is a condition known as anemia. Anemia can result from nutritional deficiencies, particularly a lack of iron, vitamin B12, or folate. Other causes include chronic diseases, such as kidney disease, or conditions that cause internal bleeding.
A higher-than-normal RBC count is referred to as polycythemia or erythrocytosis. This can be caused by the body overproducing red blood cells due to a bone marrow disorder. It is also a secondary response to conditions that create a chronic lack of oxygen, such as certain heart or lung diseases. Any abnormal blood test result should be interpreted by a healthcare provider to determine the cause.