Re‐assessment of the Reliability of the Haematocrit

S ummary . Significant quantities of plasma are trapped between the centrifuged red cells in the microhaematocrit. This produces a falsely high haematocrit and mean corpuscular volume, and a falsely low mean corpuscular haemoglobin concentration. Trapped plasma accounts for approximately 3% of the normal red‐cell column. Slightly higher values are found in macrocytic anaemias, but values as high as 5–6% may be found in the hypochromic anaemias, and even over 20% in sickled samples of blood. It is suggested that automatic blood counters of the Coulter S type be set to record a true haematocrit without trapped plasma. This value is 1 1/2% less than the microhaematocrit value with normal blood. Absolute values in normal subjects under these conditions are reported.