Determination of D-lactate by enzymatic methods in biological fluids: study of interferences

Analysis of nondeproteinized samples with an enzymatic method to determine D-lactate indicated interferences. The presence of L-lactate dehydrogenase (LD) and L-lactate in the sample led to underestimation of D-lactate content when a sample blank was processed and overestimation when it was omitted. We proved that this interference is not due to lack of D-LD stereospecificity. Moreover, assessment of D-LD and L-LD KM for NAD+ allowed us to rule out the different affinities for this coenzyme as a cause of the interference. Our results underline the importance of deproteinizing samples for D-lactate analysis when enzymatic methods are used. The ultrafiltration procedure we propose is convenient and shows acceptable mean recovery (108%) and good imprecision (within-run CV = 4.2% and 3.0% for D-lactate at 31 and 107 mumol/L, respectively; between-run CVs were 7.3% at 49 mumol/L D-lactate and 3.1% at 115 mumol/L D-lactate).