As part of a College of American Pathologists (CAP) proficiency testing survey, a comparison is made between human chorionic gonadotropin (hCG) results from an actual patient pool and a similarly targeted artificial sample. The goal is to gain insight into the possible source(s) of bias attributable to the proficiency testing material (PTM) with a view toward creating more appropriate survey materials.
To compare hCG and related variants in a pool of fresh frozen sera (FFS) with that found in PTM.
The 2003 CAP K/KN-A Survey included a FFS specimen along with admixtures of PTM. The FFS (K-02) and 1 PTM admixture (K-01) had similar mean hCG values. Five hCG-related analytes were measured on these 2 samples by a reference laboratory.
Approximately 1800 clinical laboratories and diagnostic test kit manufacturers participated in the K/ KN-A Survey.
Method imprecision (coefficient of variation) and method bias (relative difference between peer group mean and all-method median) were computed for the 2 samples. Differences were evaluated with respect to hCG-related analytes levels.
All-method hCG results were 12.9 mIU/mL (12.9 IU/L) for the PTM material and 21.6 mIU/mL (21.6 IU/L) for the FFS material. Method biases for 14 manufacturers were greater for PTM than for FFS (-40% to +35% and -16% to +23%). Twelve of 14 methods had higher coefficients of variation on PTM. Total hCG and free beta hCG measurements by the reference laboratory were 14.1 mIU/ mL (14.1 IU/L) for the PTM material and 18.5 mIU/mL (18.5 U/L) for the FFS material (FFS), and 2.4 (PTM) and 0.7 (FFS) mIU/mL (2.4 and 0.7 IU/L), respectively. On a molar basis, free beta represented 17% and 4% of the total hCG, respectively. Levels of hyperglycosylated hCG, nicked hCG, and beta core fragment were not measurable in either sample.
It is unlikely that the hCG added to the PTM is the source of the increased bias and variability. The main difference is a 3-fold increase in free beta found in the PTM, but methods previously found to strongly react with free beta were not systematically elevated. The biases between manufacturers found for the FFS specimen are likely attributable to calibration differences.