An RBC count is a blood test that tells how many red blood cells (RBCs) you have.
Why the Test is Performed
- The RBC count is almost always part of the CBCCBC (complete blood count) test.
- The test can help diagnose anemia and other conditions affecting red blood cells.
- Additional conditions under which an RBC count may be performed:
- Alport syndromeAlport syndrome
- Drug-induced immune hemolytic anemia
- Hemolytic anemia due to G6PD deficiencyHemolytic anemia due to G6PD deficiency
- Hereditary anemias, such as thalassemia
- Idiopathic autoimmune hemolytic anemiaIdiopathic autoimmune hemolytic anemia
- Immune hemolytic anemiaImmune hemolytic anemia
- Macroglobulinemia of WaldenstromMacroglobulinemia of Waldenstrom
- Paroxysmal nocturnal hemoglobinuria (PNH)Paroxysmal nocturnal hemoglobinuria (PNH)
- Primary myelofibrosisPrimary myelofibrosis
- Newborns: 4.8 - 7.2 million
- Adults: (males): 4.6 - 6.0 million
- (Females): 4.2-5.0 million
- Pregnancy: slightly lower than normal adult values
- Children: 3.8 – 5.5 million
- A blood collection and diluting system is first assembled.
- To gain access to diluting solution, the reservoir seal needs is punctured.
- A finger is lanced and a small drop of blood is allowed to accumulate.
- Blood is drawn into the pipette.
- Blood in the pipette is transferred to diluting reservoir, which contains a solution (0.9% saline) which is isotonic for blood cells and prevents them from rupturing. Usually, there is enough fluid in the reservoir to dilute the blood by a factor of 200-400 times and the dilution factor is provided by the manufacturer.
- The reservoir and pipette are converted into a dropper assembly by removing the pipette from the reservoir and reseating it in the reverse position.
- A cover slip is placed on a hemacytometer and the dropper assembly is carefull positioned next to the coverslip. A few drops of the solution are squeezed from the reservoir.
- The hemacytometer draws a set amount of solution under the coverslip. Usually this is limited to a depth of 0.1mm.
- Etched in the hemacytometer is a counting grid which appears under magnification.
- Five grid squares are used to count RBCs;
- All RBCs in each grid square are counted.
- RBCs count = RBCs counted in 5 squares*volume correction factor*dilution factor
- Correction factor = desired volume/ calculated volume
What Abnormal Results Mean
- Hemolytic anemia
- Hemorrhagic infections
- Gastrointestinal (GI) or other vascular bleed
- Iron deficiency anemia
- Vitamin B12 or folate deficiency
- bone marrow damage
- Metabolic disorders
- Chronic inflammation
- Pulmonary disease
- Congenital heart disease
- Polycythemia vera
- Renal problems
- Over-transfusion of whole blood
- Tissue hypoxia
- Polycythemia vera is a disease of unknown origin that results in an abnormal increase in red blood cells. Polycythemia vera is referred to as a "primary polycythemia" because the overproduction of red blood cells does not result from hypoxia.
- The term "vera" means true; thus polycythemia vera refers specifically to overproduction of red blood cells in the bone marrow not caused by a physiologic need.
- Polycythemia vera is treated by radioactive phosphorus to slow down bone marrow overproduction of red blood cells.
- Hydration is an important consideration when caring for patients with abnormally high red blood cell counts.
- Very high RBC mass will slow blood velocity and increase the risk of intravascular clotting.
- Examples of "secondary polycythemias", that occur in response to hypoxia, are chronic lung disease in adults and children with congenital heart defects characterized by cyanosis.
- Anemia describes the condition in which the number of red blood cells in the blood is low. Anemia is of three types depending upon the size of the red blood cells. Size is reflected in mean corpuscular volume (MCV).
- The mean corpuscular volume, or "mean cell volume" (MCV), is a measure of the average red blood cell volume that is reported as part of a standard complete blood count.
- In patients with anemia, it is the MCV measurement that allows classification as either a microcytic anemia (MCV below normal range), normocytic anemia (MCV within normal range) or macrocytic anemia (MCV above normal range).
- It can be calculated (in litres) by dividing the hematocrit by the red blood cell count (number of red blood cells per litre). The result is typically reported in femtolitres.
- If the MCV was determined by automated equipment, the result can be compared to RBC morphology on a peripheral blood smear. Any deviation would be indicative of either faulty equipment or technician error.
- For further specification, it can be used to calculate red blood cell distribution width (RDW).
The normal reference range is typically 80-100 fL.
In presence of hemolytic anaemia, presence of reticulocytes can increase MCV. In pernicious anemia (macrocytic), MCV can range up to 150 femtolitres. An elevated MCV is also associated with alcoholism (as are an elevated GGT and a ratio of AST:ALT of 2:1). Vitamin B12 and/or Folic Acid deficiency has also been associated with macrocytic anemia (high MCV numbers).
- The most common causes of microcytic anemia are iron deficiency (due to inadequate dietary intake, gastrointestinal blood loss, or menstrual blood loss), thalassemia, or chronic disease.
- A low MCV number in a patient with a positive stool guaiac test (bloody stool) is highly suggestive of GI cancer.
- In iron deficiency anemia (microcytic anemia), it can be as low as 60 to 70 femtolitres. In cases of thalassemia, the MCV may be low even though the patient is not iron deficient.
- If the red blood cells are smaller than normal, this is called microcytic anemia. The major causes of this type are iron deficiency (low level iron) anemia and thalassemia (inherited disorders of hemoglobin).
- If the red blood cells size are normal in size (but low in number), this is called normocytic anemia, such as anemia that accompanies chronic disease or anemia related to kidney disease.
- If red blood cells are larger than normal, then it is called macrocytic anemia. Major causes of this type are pernicious anemia and anemia related to alcoholism.