Does a raised urea concentration in serum interfere with the haemoglobin value in semi automated kinetic methods?
A. The classical method of choice for haemoglobin (Hb) measurement is the Drabkin’s method in which Hb is released from the RBCs by hypotonic lysis (automated methods add surfactants to speed the lysis), the heme iron is then oxidized to the ferric form (Fe+++) using potassium ferricyanide, and the iron reacts with the cyanide anion of potassium cyanide giving rise to a distinctive stable chromogen with a spectrophotometric peak at about 540 nm. Automated analyzers generally modify the Drabkins method by using a cationic surfactant at pH above 10 in the presence of cyanide to lyse red cells so that white cell nuclei remain intact and can be counted by impedance methods. Errors can arise if the sample is turbid (lipemic or highly icteric) or has hyperleukocytosis (very high white cell count) because of scattering of light by the nuclei or increased absorbance due to lipids or bilirubin. High urea levels have no effect.
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