Some aspects of residual formaldehyde testing when reusing haemodialysers

To assess the adequacy of quantifying residual formaldehyde concentrations when reusing, four semi quantitative methods of concentration estimation (Clinitest tablets, Schiff's reagent, Formalert Formotest) were compared. Two of these methods (Schiff's reagent and Clinitest) were inadequate in detecting low concentrations of formaldehyde and were associated with false positives from interference by chemicals contained in the dialysate. When using the two reagents specifically manufactured for this purpose, we also observed false positives with one (Formotest) while the other was capable of detecting formaldehyde concentrations as low as 4.5 mg/l. To minimise patients' exposure and the formation of anti-N like antibodies resulting from formaldehyde damage of red cells, it would be prudent when reusing, to select the most sensitive of these semi-quantitative techniques for routine use, to perform regular screening for anti-N antibodies and to periodically check formaldehyde levels by the use of the highly specific Hantzsch reaction.