In pregnant women, it is occasionally necessary to be able to decide if a clear fluid is amniotic fluid or urine.
Make a smear of the fluid on a glass slide.
(1) If "ferning" is seen, then it represents amniotic fluid (unless a missed abortion has occurred).
(2) If no "ferning" is seen, then the fluid likely is not amniotic fluid.
Obtain a known urine specimen from the patient. The unknown fluid should give similar chemical findings if it is urine.
If no urine sample is available, the following features can be assessed. A blood sample for simultaneous analysis is useful.
Analyte
Urine
Amniotic Fluid
bilirubin
negative
may be present
creatinine
high (typically >= 20 mg/dL)
similar to plasma (0.8 - 4.0 mg/dL)
osmolality
500-800 mOsm per kg water
230-295 mOsm per kg water
pH
usually acidic
neutral or alkaline (6.91-7.43)
protein
usually absent (1-14 mg/dL)
present (70 - 840 mg/dL)
specific gravity
1.005-1.030
1.025
urea
high (order of 500-1000 mg/dL)
similar to plasma (12.1 - 41.7 mg/dL)
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Specialty: Hematology Oncology, Clinical Laboratory, Pulmonology, Surgery, general, Nephrology
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