Amniotic fluid creatinine is used to evaluate fetal maturity as its concentration progressively increases as pregnancy advances. Creatinine, a byproduct of muscle metabolism found in amniotic fluid, reflects increased fetal muscle mass and the ability of the maturing kidney (i.e., glomerular filtrating system) to excrete creatinine into the amniotic fluid. The patient’s blood creatinine level should be known before the amniotic fluid creatinine value is interpreted.

Creatinine indicates fetal physical maturity and correlates reasonably well with the level of lung maturity. Normal lung development is dependent on normal kidney development. As pregnancy progresses, the amniotic fluid creatinine level increases. A value of 2 mg/dL (177 μmol/L) is accepted as an indicator that gestation is at 37 weeks or more. However, the use of this value alone to assess maturity is not advised for several reasons. A high creatinine concentration may reflect fetal muscle mass but not necessarily kidney maturity. For example, a large fetus of a patient with diabetes may have high creatinine levels because of increased muscle mass. Conversely, a hypertensive patient who has a small infant with growth restriction may have low creatinine levels because of decreased muscle mass. Creatinine levels can be misleading if they are used without other supporting data. So long as maternal blood creatinine levels are not increased, amniotic fluid creatinine measurements have a certain degree of reliability if they are interpreted in conjunction with other maturity studies.

1. Obtain an amniotic fluid sample of at least 0.5 mL.

2. Protect the specimen from direct light.

3. Obtain a maternal venous blood sample.

Creatinine levels lower than expected may occur in the following situations:

1. Early in the gestational cycle (not yet at 37 weeks)

2. Fetus smaller than normal (growth restriction)

3. Fetal kidney abnormalities

4. Prematurity

Pretest Patient Care

1. Explain the purpose of the test.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Causes of elevated amniotic fluid creatinine concentrations that are not consistent with gestational age include abnormal maternal creatinine, diabetes, and preeclampsia.

Normal

• A value >2 mg/dL or >177 μmol/L indicates fetal maturity (at 37 weeks) if maternal creatinine is normal.