Serum cystatin is not a marker of glomerular filtration rate in pregnancy.

The role of cystatin C (Cys-C) as a marker of glomerular filtration rate (GFR) in pregnancy is undetermined. Measurements of Cys-C and creatinine (Cr) were taken at 14-17(+6), 18-23(+6), 27-31(+6) weeks' gestation, at delivery and 2-6 weeks postpartum in a prospective observational study of 27 women. There was no difference between Cys-C levels in early and late second trimester, but they were significantly higher in early third trimester (P < 0.001) than second trimester, despite no concurrent increase in Cr. Cys-C was also significantly higher at delivery than at all other times in pregnancy (P < 0.001) and fell to postpartum values higher than second trimester measurements (P < 0.01), but lower than delivery (P<0.001). In conclusion, changes in Cys-C may be influenced by pregnancy-related changes in glomerular filtration and therefore we would advise against their use as a marker of GFR in pregnancy.