Creatinine increases in amniotic fluid over gestation due to the increased fetal muscle mass. The results correlate with gestational age but not necessarily with fetal lung maturity.
Specimen: Amniotic fluid, protected from light
Testing: Chemically, as for serum.
Interpretation:
• > 2 mg/dL: indicates 40+ weeks of gestation
Limitations:
• requires a certain level of renal function to provide creatinine excretion
• small for gestation fetuses have less muscle mass and so may give low values
• high maternal creatinine from renal disease can diffuse into the amniotic fluid
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