What does it measure?
Red blood cells pick up oxygen in your lungs and deliver it to your tissues. This test counts the cells in a blood sample. It is part of a complete blood count, or CBC, and is most useful beside hemoglobin and the size of your red cells. [1]
Before the test
- Sample
- Blood, usually collected with a complete blood count.
- Fasting
- The CBC alone usually needs no fasting. Other tests at the appointment may have different instructions.
What can the result mean?
A low count can reflect blood loss, reduced cell production, or cells breaking down sooner than expected. A high count can reflect dehydration or a response to lower oxygen levels. The number alone cannot tell which explanation fits you. [1]
When is it useful, and how is it read?
When anemia is being assessed, read the RBC count beside hemoglobin and MCV. Small cells with a preserved or higher count suggest a different pattern from many cases of iron deficiency; history and confirmatory testing decide the cause. [4] [5]
What can affect it?
Pregnancy, living at altitude, smoking, and hydration can change the result. Your own previous counts help show whether a change is new. The CBC itself usually needs no fasting; other tests taken at the same appointment may have different instructions. [1]
More cells can still mean smaller cells
A relatively high red cell count can appear with thalassemia trait, even when the cells are small. The count alone does not tell you how much oxygen-carrying hemoglobin is present. [4] [5]
Questions you might have
Is RBC the same as hemoglobin?
No. RBC counts the cells; hemoglobin measures the oxygen-carrying protein in your blood. Both help explain the same blood picture. [3]
Can dehydration raise the count?
Yes. Less fluid can make cells more concentrated in the sample. A high count needs the hydration context and previous results. [1]
What comes next?
Bring earlier results and mention recent bleeding, heavy periods, breathlessness, or new medicines. Your clinician may look at iron stores, kidney function, or young red cells before deciding what needs attention. Iron supplements are useful only when the cause calls for them. [1]
How does this count fit with my hemoglobin, cell size, and previous results?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
Further reading
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
- Provan D, editor. Oxford Handbook of Clinical and Laboratory Investigation. 4th ed. Oxford University Press; 2018.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
