The haematocrit (Ht, HCT, packed cell volume, PCV, erythrocyte volume fraction, EVF) is the fractional volume of red blood cells in blood expressed as a percentage(%). It is a less sensitive index of anaemia in clinical practice than alternatives. Produced by standard coulter counters, it is derived from other values and reflects both the number of red blood cells and the size of red blood cells, indeed being RBC multiplied by the MCV. The packed cell volume can also be determined by centrifuging unclotted blood, and this was how it was determined before automatic counters with a standard methodology usually using microcapillary tubes spun at 10,000 revs for 5 minutes, that separated the red cells from the plasma. Indeed this is a low technology way to determine anaemia in say rural settings and as such, even though it is less precise than using haemoglobin level to define anaemia it is far more practicable in much of the third world. The haematocrit is an indirect measure of the oxygen carrying capacity of the blood. It will be influenced more than a direct measure like haemoglobulin concentration by plasma volume changes such as dehydration.
When whole blood is allowed to clot, the formation of clot consumes clotting factors. Centrifugation of the sample forces the clot to the bottom of the tube, leaving a pale straw-coloured fluid called serum. If anti-coagulant is added to the sample before centrifugation, the resulting fluid is known as plasma. Plasma differs from serum in that in still contains unused clotting factors.