[Analytical interference of lipid parenteral feeding with the determination of methemoglobin levels by an...

We report here an unusual cause of analytical interference observed in methemoglobin values of patients parenterally given nutritive lipid emulsions. In these patients, harboured in intensive care department, T Hb, % O2 Hb, % COHb, Met Hb values are systematically measured using IL 282 CO-Oximeter autoanalyser in addition to daily blood gas determination (pH, PaCO2, PaO2). An increase in % Met Hb rate up to 10-20% was observed in lipid emulsions receiving patients. We first verified that washing red blood cells with saline instantaneously lowers Met Hb values to less than 1%. The mechanism of this interference was addressed by performing three Met Hb determination methods in normal blood in vitro added with variable amounts of lipid emulsions: CO-Oximeter determination, classical Evelyn Malloy method, analysis of continuous absorption spectra between 480 and 640 nm. Results corroborate the spectral origin of the analytical interference observed with CO-Oximeter and leading to false positive values. Blood lipids increase unspecifically the wave length proportional absorption between 480 and 640 nm. Evelyn Malloy's technique suppresses this interference since it uses the ratio of differences in optical densities. Our results emphasize the necessity of knowing patients therapeutics when performing laboratory investigations.