Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (279 page)

   
Fetal fibronectin
in cervical secretions >50 ng/mL (immunoassay) or rapid test identifies women who deliver before term with S/S = 60–93%/52–85%, PPV = 25%. With high-risk patients, S/S = 70/75%. NPV = 96% rules out labor within 7 days
^1,2
. Normally present in early pregnancy and within 1–2 weeks of onset of labor at term but normally absent from cervicovaginal fluid after 20 weeks. It is also present in AF so testing after rupture of membranes is not helpful. If present between 24 and 36 weeks, it precedes preterm labor/ birth by ≥3 weeks.

   Laboratory findings are due to associated conditions (e.g., hyaline membrane disease, intraventricular hemorrhage).

References

1.  Sanchez-Ramos L, Delke I, Zamora J, et al. Fetal fibronectin as a short-term predictor of preterm birth in symptomatic patients: a meta-analysis.
Obstet Gynecol
2009;114:631.

2.  Honest H, Bachmann LM, Gupta JK, et al. Accuracy of cervicovaginal fetal fibronectin test in predicting risk of spontaneous preterm birth: systematic review.
BMJ.
2002;325:301.

RUPTURED MEMBRANES

   Definition

The diagnosis of rupture of membranes is best made on direct observation of fluid leaking from the cervical os. Laboratory diagnosis of fluid from the posterior fornix as amniotic fluid (AF) rather than urine may be necessary.

   Laboratory Findings

Laboratory methods for detecting AF in the vagina:

   The “Fern” test is the most reliable test (>96% accuracy). AF air-dried on glass slide shows a characteristic fern-like pattern microscopically. Results are false positive in the presence of cervical mucus or semen and false negative in the presence of blood, dry swab, or insufficient drying time; they are not affected by meconium or pH.

   Amniotic fluid pH is 7.0–7.3 while normal vaginal pH is 3.8–4.2. The nitrazine paper test changes from blue to yellow if the pH is >6.5, with accuracy approximately 93%
¹
. Results are false positive due to blood, semen, alkaline urine, trichomoniasis, and bacterial vaginosis. A reagent strip test pH ≥7 and protein ≥100 mg/dL indicate the presence of AF (Table 8-2).

   Placental alpha microglobulin-1 protein assay point of care test using immunochromatography to detect trace amounts of placental alpha microglobulin-1 protein in vaginal fluid
²
. Compared to fern testing or nitrazine paper testing, the cost of this test is significantly higher and should be limited to use when the diagnosis remains uncertain following the previous tests.

   Measurement of AFP in vaginal secretions is unreliable; same concentration in AF and maternal plasma in the third trimester.

References

1.  Abe T. The detection of rupture of fetal membranes with the nitrazine indicator.
Am J Obstet Gynecol.
1940;39:400.

2.  Abdelazim IA, Makhlouf HH. Placental alpha microglobulin-1 (AmniSure(®) test) for detection of premature rupture of fetal membranes.
Arch Gynecol Obstet.
2012;285:985.

TOXEMIA OF PREGNANCY (PREECLAMPSIA/ECLAMPSIA)

   Definition

Preeclampsia is characterized by hypertension, proteinuria, and edema (of the face, hands, and legs) after the 20th week of pregnancy. It is a multisystem disorder and when severe will show signs of end-organ injury. Eclampsia refers to the new onset of seizures in a patient with preeclampsia. The incidence of preeclampsia is ≤7.5% of pregnancies worldwide.
¹
The etiology is unknown and likely involves maternal and fetal/placental factors. There is abnormal placental vasculature early in pregnancy, which may result in underperfusion, hypoxia, and ischemia. This may lead to circulating antiangiogenic factors that cause maternal endothelial dysfunction resulting in hypertension and proteinuria.
²

   Laboratory Findings