On the determination of treponeme immobilizing potency of weakly reactive sera and its suggested interpretation.

Methods previously developed for determination of the TPI potency of strongly reactive sera have required modification to permit confidence in estimating potency in weakly reactive sera so as to correct for apparent blank reactivity and for possible inhibition by other serum constituents when a high enough serum concentration is used to permit detection of TPI activity. The intrinsic or blank potency (w') and the relative sensitivity (f) in the presence of a given amount of specimen serum are estimated separately, the latter through use of an internal standard by adding a convenient definite amount of standard serum. A primary aim of this study was to find what value of potency (p) would not be exceeded, or rarely so, by reactions of sera that had previously been evaluated as "negative" on a crude qualitative basis; it seems that p should be 1 or less for all such specimens. Those rated previously as "positive" appear almost always to have p greater than 3. Those with p in the interval from 1 to 3 indicate a need for further tests, preferably with fresh specimens.