Temporal changes of the plasma levels of cystatin C, beta-trace protein, beta(2)-microglobulin, urate and creatinine during pregnancy indicate continuous alterations in the renal filtration process

Objective. To determine the plasma levels of the renal functional markers creatinine, urate, cystatin C, beta(2)-microglobulin and beta-trace protein in samples from the first, second, early third and late third trimesters of 398 healthy women with uncomplicated singleton pregnancies. Material and methods. Plasma samples from 58 healthy non-pregnant women served as controls. The creatinine levels were significantly lower at all time-points in pregnancy, whereas the urate levels were lower during the first and second trimesters but increased in the late third trimester. The cystatin C, beta(2)-microglobulin and beta-trace protein levels displayed similar changes with increased levels in the third trimester but unaltered levels during the first and second trimesters. Results. The results indicate an increased filtration of low-molecular weight molecules during pregnancy, particularly during the first and second trimesters, whereas filtration of 10-30 kDa molecules is decreased in the third but unaltered in the first and second trimesters. The levels of albumin and alpha(2)-macroglobulin were measured in the same samples. Conclusions. The albumin levels decreased in the second and third trimesters, whereas the levels of alpha(2)-macroglobulin were unchanged, which is compatible with a virtually unaltered transfer of alpha(2)-macroglobulin between the intra-and extravascular space during pregnancy and a significantly increased extravascular fraction of albumin.