We have blogged previously (here and here) about false negative results in qualitative point-of-care (POC) hCG devices due to high concentrations of hCGβcf in urine. We have shown that the majority of qualitative POC hCG devices are actually susceptible to false negative results due to saturation of capture antibodies by high concentrations of hCGβcf. These findings have led to an increased interest in urine hCGβcf measurements.
However, only a few research laboratories quantify hCGβcf, so it is difficult for most clinical laboratories to determine which urine samples have high concentrations of this hCG variant.
Previously, we demonstrated that the Roche Cobas hCG+β assay detects hCGβcf but the Abbott Architect Total β-hCG assay does not. Using that information, in a recent study, we examined the correlation between hCGbcf concentrations as measured by LC-MS/MS and the absolute difference between urine hCG measurement using the Roche Cobas and Abbott Architect assays in fourteen urine samples. The correlation between hCGβcf concentration measured by LC-MS/MS and the Roche-Abbott difference was excellent (r=0.97).
This study indicates that the difference in urine hCG concentration when measured by two automated immunoassays (one that recognizes hCGβcf and one that does not) provides a robust estimate of the urine hCGβcf concentration. Using the linear regression equation (Roche-Abbott difference (in IU/L) = (hCGβcf (pmol/L)*0.131+656)), a urine hCGβcf of 280,000 pmol/L translates into an approximate Roche-Abbott difference of 37,336 IU/L.
There are many unanswered questions about hCGβcf, including: How high do hCGβcf concentrations get during pregnancy? Are some women prone to higher hCGβcf concentrations? Are high hCGβcf concentrations associated with certain clinical symptoms or pathological conditions? The ability to estimate the amount of hCGβcf in urine will allow many more laboratories to study the concentrations of this interesting hCG variant.