How comparable are total human chorionic gonadotropin (hCGt) tumour markers assays?

Total human chorionic gonadotropin (hCGt) tumour marker testing is regarded as an "off label" application for most commercial methods. We compared four assays in patients with a hCGt tumour marker request. We hypothesised that regression slopes would be altered and that outliers would be more common with tumour marker than with pregnancy samples if the detection of malignancy associated hCG molecular forms differed amongst assays. Further such systematic differences would be obvious and large enough to change clinical management decisions. We measured hCGt in 390 samples from 137 females and 253 males with a tumour marker request and 208 pregnancy controls with the following methods: Access Total βhCG, Architect Total-βhCG, Cobas hCG + β and Immulite HCG. The between method regressions determined on tumour marker and pregnancy samples were not significantly different. The outlier rates were similar for male and female tumour marker and the pregnancy groups: 1.6% (95% confidence interval [CI] 0%-3.1%), 2.2% (95% CI 0%-4.7%) and 2.9% (95% CI 0.6%-5.2%). The outliers were randomly distributed amongst the methods and we were confident that they would not adversely influence clinical decisions. The hCGt results were clinically equivalent with no systematic difference amongst the four assays