Use of formaldehyde-PBS for serum dilution in the microscopic agglutination test (MAT) for leptospirosis.

Altogether, 431 sera (381 positive and 50 negative sera) were tested against different Leptospira serovars in the microscopic agglutination test (CMAT) using PBS with and without formaldehyde for comparative purposes. For the preparation of serum dilutions with formaldehyde-PBS, formaldehyde was added to PBS at a final concentration of 0.4%. When retested after storage, 234 from the 381 formerly positive sera gave positive, 68 doubtful and 79 negative results in the MAT. Out of the 234 sera with positive reaction in MAT, 212 (90.6%) showed positive reactions in the MAT with formaldehyde as well, 19 (8.1%) doubtful reactions and 3 (1.3%) became negative. All sera with negative reaction in the routine MAT were found to be negative in the MAT with formaldehyde as well.