Conventional ROM Testing Methods vs. AmniSure®Testing for premature rupture of membranes (PROM) during pregnancy dates back to the 1940s. Amino-dye infusion testing, a current gold standard, is highly invasive, expensive and requires specialized expertise and equipment to administer. Conventional ROM testing methods, such as nitrazine pH, pooling, and ferning, offer fewer barriers to deployment. In recent years tests based on biomarkers have been introduced into the market.
Pooling entails the visualization of an amniotic pool in the posterior fornix of the vagina. This procedure requires a sterile speculum exam to observe the vaginal cavity, is subjective and fluids such as urine and semen can be mistaken for amniotic fluid and vice versa. Because 40-47% of ROM cases do not have visual leakage of amniotic fluid form the cervical os, other diagnostic tests may be necessary for an accurate diagnosis.
The Fern test for ROM diagnosis is a microscopic observation of a dried vaginal specimen to determine the presence of a fern-like pattern, or crystallization, known to be indicative of amniotic fluid.
A speculum exam is required to collect the sample, and a microscope is required to observe the sample. False positives may result from contamination of the microscope slide with fingerprints, semen, or cervical mucus. False negative results may be caused by dry swabs, or interference by either blood or other cervicovaginal discharge. False positive and false negative results can be experienced in 5-30% and 12.9% of cases, respectively. A practical limitation of the fern test may be the lack of certified personnel available to read the microscope slides.
The strength of the fern test is that it is able to differentiate between amniotic and non-amniotic fluid in vaginal discharge, however, when it was comparatively investigated in laboring and non-laboring patients there were noted differences in sensitivity and specificity. Sensitivity and specificity in the laboring group were 98.0% and 88.0%. In the non-laboring group, sensitivity and specificity were only 51.4% and 70.8%, respectively. .
Typically, cericovational fluid will have a pH of between 4.5 and 5.5. Amniotic fluid is far more alkaline — as a result, abnormal pH levels can be an indicator of PROM. The nitrazine test for ROM during pregnancy offers increased sensitivity when compared to ferning. It can also be useful for detecting PROM in the ~47%1 of cases where amniotic pooling is not visible during a speculum examination.
However, nitrazine testing suffers from a high degree of subjectivity when interpreting results. Many substances can interfere with the test including bacterial vaginosis or Trichomonas, alkaline urine, blood, semen, or antiseptics, potentially leading to false diagnoses and unnecessary interventions. In certain investigations of nitrazine testing, specificity has been reported to be as low as 16%2. A speculum exam is also required to perform the test. False-positive results of the nitrazine test may be caused by cervicitis, vaginitis. False positive and false negative results of the nitrazine test are up to 17.4% and 12.9%, respectively with sensitivity of 90.7% and specificity of 77.2%.
- Neil PRL, Wallace EM. Is AmniSure® useful in the management of women with prelabour rupture of the membranes? Australian and New Zealand Journal of Obstetrics and Gynaecology 2010.
- Erdemoglu E, Mungan T. Significance of detecting insulin-like growth factor binding protein-1 in cervicovaginal secretions: comparison with nitrazine test and amniotic fluid volume assessment. Acta. Obstet. Gynecol. Scand. 2004;83:622–6.