Introduction
This topic covers the basic hematology screen that almost every patient receives: the complete blood count (CBC), the differential blood count (DBC) and the erythrocyte sedimentation rate (ESR). In the exam you should be able to name each measured parameter, quote approximate adult reference ranges, state when each test is indicated, and interpret abnormal results as a starting point for further investigation rather than as a final diagnosis.
Specimen and general principles
The CBC and DBC are performed on venous blood anticoagulated with EDTA, analysed automatically on a hematology analyser and checked by a smear when flags are raised. The ESR is measured on citrated blood, most often by the Westergren method, and is read after one hour. Reference ranges depend on age, sex, laboratory method and, for some parameters, altitude, so the student should always use the reference range printed by the laboratory that produced the result.
Red cell indices
The analyser calculates the red cell indices, which describe the size and hemoglobin content of the average erythrocyte. They are used to classify anemias and should always be read together with the Hb value.
- MCV (mean corpuscular volume): about 80–100 fL. Below 80 fL is microcytic; above 100 fL is macrocytic.
- MCH (mean corpuscular hemoglobin): about 27–33 pg, with limits varying between laboratories.
- MCHC (mean corpuscular hemoglobin concentration): about 320–360 g/L. A low value means hypochromia.
- RDW (red cell distribution width): about 11.5–14.5%. A raised value means anisocytosis, that is, a mixed population of cells of different sizes, which often appears early in iron or B12 or folate deficiency.