Stability of hCG free -subunit and -core fragment in urine Recently, Spencer et al. (1996) tested maternal urine free -subunit levels in second-trimester Down syndrome pregnancies. Urine free -subunit levels were 2•6-fold elevated (2•6 MOM) in Down syndrome cases. A detection rate of 58 per cent was indicated at a 5 per cent false-positive rate. Urine free -subunit was seen as a possible improvement over serum assays, more effective, and less invasive or more easily acceptable in some cultures.
How stable is urine free -subunit? To date, nothing has been published about its stability in urine. This issue must be assessed before data on urine free -subunit can be considered seriously. Instability of free -subunit in serum samples is primarily due to dissociation of hCG. -subunit released by dissociation may swamp free -subunit levels (<0•6 per cent of hCG level). We used two independent methods, dissociation of added hCG and changing levels in parallel pregnancy serum and urine samples, to assess the stability of free -subunit samples. Pure hCG batches P8 and P9 (not nicked, with standard N-and O-linked oligosaccharides) were added to parallel pools of six fresh serum or six fresh urine samples from non-pregnant individuals (final concentration 1 g/ml or 9•3 IU/ml). Preservative (2X pen-strep-fungizone, GIBCO, Grand Is., New York, U.S.A.) was added to maintain sterility and samples were incubated up to 4 weeks at 37 C. Dissociation of hCG was determined by the release of free -subunit, as measured by immunoassay (Cole et al., 1993). As shown in Fig. 1a, very similar results were found with hCG P8 and P9 (no significant difference). The rate of dissociation was 15 1•6 per cent per week in serum and 32 4•1 per cent per week in urine (urine vs. serum, P=0•001 by Student's t-test). Similar results have been recorded by Dr R. Iles and S. Butler at St Bartholomew's Hospital, London, U.K. (personal communication). These results indicate that hCG dissociates more than two times faster in sterile urine than in sterile serum samples.