Determination of haematocrit using the SMA4.
A E Green, A. Gordon Signy · Journal of Clinical Pathology · 1968
We were interested to read the letter from Drs Lappin and Lamont (1968) describing the effect of EDTA on the haematocrit as estimated by the electrical conductance method using the SMA4 machine.In our early trials of the SMA4, the haematocrit line gave the most trouble.We found that with the strict quality control procedure already in use in the laboratory, the haemoglobin, white cell count, and red cell count results were acceptable for routine use.Each morning 15 ml of blood is taken from an outpatient into EDTA and assayed for haemoglobin using the cyanmethaemoglobin method which is standardized with the International Standard.The white cell and red cell counts are estimated by electronic particle counting and the haematocrit by microcentrifugation.This blood is used to standardize and to phase the SMA4 and inserted as every twentieth sample throughout the day.It soon became apparent that the haematocrit line gave a small percentage of erratic results.As the effect of excess EDTA on the centrifuged haematocrit has been demonstrated (Lampasso, 1965), we discard all inade- quately filled specimen bottles.Several other factors are peculiar in affecting the elec- trical conductance method.Kernen, Wurzel, and Okada (1961), in the original evaluation of the method, emphasized that blood conductance is sensitive to temperature variation and regarded the inclusion of a thermistor in the circuit as essential.As no thermistor is included in the SMA4 we maintain ambient temperature as constant as possible.Kernen et al. ( 1961) also showed that the calibration is not linear.To achieve maximal linearity we standardize the SMA4 machine with two potassium chloride solutions, 5 07 and 2-66 g/l, equivalent to haematocrits of 19% and 50% respectively.Even so, discrepancies still occur outside the range 15-55%.Davis, Bresland, and Green (1966) and Kernen et al. (1961) drew attention to the effect of high plasma protein levels (exceeding 7 g/100 ml).In one week we found that of 41 protein estimations 15 were abnormal but only one was a high value, the rest being low.In practice we have found the high protein levels in multiple myelomatosis and macroglobulinaemia to increase the haematocrit reading.The same authors showed the effect of alterations in sodium concentration.Again, in one week, of 166 sodium estimations we found 7 % outside the normal