Fetal serum concentrations of cystatin C and β₂-microglobulin as predictors of postnatal kidney function

OBJECTIVES: Cystatin C and β₂-microglobulin are established serum markers of renal function in children and adults. In contrast to creatinine, diaplacental exchange is minimal. The aim of the study was to establish reference values in fetal serum and to test their efficiency in predicting postnatal kidney function. STUDY DESIGN: This was a prospective noninterventional study measuring cystatin C and β₂-microglobulin by particle-enhanced immunoturbidimetry in excess serum from 129 cordocenteses performed in 84 fetuses. Reference intervals (mean ± 1.96 SD) were calculated in a subgroup of 54 fetuses without evidence of kidney disease, and these reference values were evaluated in 75 sera from 55 fetuses. RESULTS: Mean cystatin C was 1.66 ± 0.202 mg/L (upper limit 2.06), and mean β₂-microglobulin was 4.25 ± 0.734 mg/L. Unlike cystatin C, β₂-microglobulin decreased significantly with gestational age so that the upper reference limit was 7.19-0.052 x gestational age in weeks. β₂-Microglobulin had higher sensitivity (90.0% vs 63.6%) and cystatin C a higher specificity (91.8% vs 85.5%) for the prediction of impaired renal function; diagnostic efficiency was equal (87.6% vs 86.1%). Fetuses with impaired renal function at birth or who were aborted for renal malformations had higher cystatin C concentrations than those in a control group. β₂-Microglobulin was increased only in fetuses who were aborted. CONCLUSION: Fetal serum cystatin C and β₂-microglobulin concentrations may be useful predictors of postnatal kidney function.