Use of beta-lactamase to hydrolyse interfering antibiotics in vitamin B12 microbiological assay using Lactobacillus leichmannii.

The presence of certain antibiotics in patients' sera can invalidate microbiological assays for vitamin B12 using Lactobacillus leichmannii by inhibiting organism growth. Analysis of 3291 consecutive serum vitamin B12 assays in a large general hospital service showed inhibition of growth in 303 samples (9.2%). In nearly all cases the interfering substance was a beta-lactam antibiotic. The value of using a beta-lactamase preparation to eliminate antibiotic effects was studied using antibiotic-containing patient sera and also normal serum to which antibiotics were added. In-vitro additions of penicillins, cephalosporins, and other antibiotics to normal serum resulted in varying levels of organism inhibition, but excellent recovery of a control value after treatment with beta-lactamase (greater than 98%). Patient sera showing inhibition were reassayed in duplicate following treatment with beta-lactamase (n = 200), the overall effectiveness of the treatment being verified by the excellent recovery (greater than 98%) of a cyanocobalamin 'spike' added to a control. In only 0.2% of cases (6 of 3291 samples) was this treatment ineffective and it has thus proved an efficient method of eliminating the problem of antibiotic interference in the vitamin B12 microbiological assay.