Reproducibility of TPI and TPCF Tests

Abstract

The Treponema pallidum immobilization (TPI) test reproduced results of initial tests to a significantly higher degree than the Treponema pallidum complement-fixation (TPCF) test did in 2 different studies. It is confirmed that the weakly reactive (WR) category is the most difficult to reproduce. The TPI test, however, is better suited to reproducing results of this type than is the TPCF test. This is of considerable importance, inasmuch as more serums tested by the TPCF method yielded WR results than the same specimens tested by the TPI method. Complete disagreements (reactive to negative, or negative to reactive) were encountered in 15 of the TPCF repeat tests. No complete disagreements were obtained in TPI repeat tests. Paired specimens tested in the same run revealed 100% agreement in the TPCF test and 97.5% agreement in the TPI test. Pairs disagreeing in the TPI test were borderline reactors, i.e. WR to N (nonreactive), or R (reactive) to WR. In 43 satisfactory TPI runs, quantitative control serums reproduced at the 50% immobilization end-point within limits of plus or minus 1 dilution.