Introduction
Premature rupture of membranes (PROM) is a significant complicating factor in contemporary obstetric practice.
The amniotic membrane protects the foetus and intact, healthy foetal membranes encourage an optimal pregnancy outcome. Biochemical changes that occur naturally during or just prior to labour reduce the integrity and elasticity of the membranes, rendering them vulnerable to rupture and thus contributing to the initiation of labour.
Once a rupture has occurred, the mother-to-be is at risk of going into labour irrespective of gestational age, and has an increased risk of developing infection. PROM is defined as the spontaneous rupture of membranes prior to the onset of actual uterine contractions, and accurate diagnosis is crucial to allow timely and appropriate intervention.
In the UK, pre-term premature rupture of membranes (PPROM) complicates only two percent of pregnancies, but is associated with 40% of preterm deliveries and can result in significant neonatal morbidity and mortality. PPROM is associated with three causes of neonatal death: prematurity, sepsis and pulmonary hypoplasia and there are maternal risks associated with chorioamnionitis.1 Timely and accurate diagnosis allows for gestational age-specific interventions to optimise perinatal outcomes and minimise the risk of potential complications to both the mother and the foetus.
This article aims to provide an overview of the various diagnostic options available to clinicians and their comparative reliability in providing an accurate diagnosis of rupture of membranes (ROM). We are indebted to Dr Ross McQuivey, MD, for his clinical input.
Conventional Clinical Assessments
The most commonly used conventional diagnostic for PROM is the sterile speculum examination (SSE).
Pooling
A speculum is used to visually determine the presence of a pool of amniotic fluid in the posterior vaginal fornix which is highly suggestive of amniorrhexis (rupture of membrane). The efficacy of this method is compromised in cases where the leak of amniotic fluid is small, and the presence of vaginal infections such as cervicitis or vaginitis can lead to high false positive diagnoses.
Nitrazine
The Nitrazine/pH Test requires the use of a swab to collect cervicovaginal secretions. Nitrazine paper is used to confirm only a change in the pH levels of cervicovaginal secretions within the naturally acidic environment of the vagina, and is associated with high false positive rates related to cervicitis, vaginitis, alkaline urine, and contamination with blood, semen or antiseptic agents.2 As a result the sensitivity of the Nitrazine test ranges from 90-97% and the specificity from 16-70%.3
Ferning
The Fern Test requires the collection of cervicovaginal secretions which are then dried and examined under a microscope. The presence of amniotic fluid is determined by the appearance of a characteristic crystal pattern. False positives have been recorded as a result of fingerprints, and contamination with blood, semen or cervical mucus. The Fern Test has a reported sensitivity of 51% for women not in labour, and a specificity of 70%.3
Ultrasound
Although ultrasound alone is not diagnostic, it may be used to support a diagnosis as oligohydramnios (low amniotic fluid) is highly suggestive of ROM when combined with a characteristic history. Ultrasound, however, is unable to detect small reductions of amniotic fluid, and marked reductions in levels may result from other clinical factors such as severe uteroplacental insufficiency.
Amnio-Dye Test
Once considered the gold standard for diagnosing ROM, the Amnio-Dye test requires an invasive amniocentesis and injection of a blue dye into the amniotic fluid. ROM is confirmed by placing a tampon in the vagina and observing the tampon for the presence of blue dye. Risk factors include infection, miscarriage, rupture of membranes, cramping and vaginal bleeding and the Amnio-Dye Test is seldom used in contemporary medical practice.
The Implications of False Positives and False Negatives
False positives occur when the diagnostic methodology indicates ROM when the membranes of the mother-to-be are in fact intact. The lower the specificity of the diagnostic method being utilised, the higher the likelihood of false positives. Traditional methodologies for detecting ROM have high levels of false positives and false negatives, as a result of their inability to differentiate amniotic fluid from other possible interfering components such as blood, urine, semen, vaginal mucus, and infection.
While false positives do not jeopardise the safety of either the mother-to-be or the foetus, they may lead to the mother-to-be being unnecessarily admitted for observation. As well as the potential cost implication to the NHS and the inconvenience of admission to the patient, false positives may impact the psychological well-being of the mother-to-be.
False negatives occur when the diagnostic methodology fails to detect ROM when the membranes of the mother-to-be have in fact ruptured. The lower the sensitivity of the diagnostic method being utilised, the higher the likelihood of false negatives. False negatives are clinically a much more significant issue than false positives, as they may lead to failure to implement appropriate interventions to minimise potential risk factors to both the mother-to-be and the foetus.
It is evident that diagnostic accuracy is critical in determining timely and appropriate gestational age-specific interventions. Traditional diagnostic methodologies may lead to misdiagnosis as they are complicated by factors beyond the simple presence or absence of amniotic fluid in the vaginal vault, which has led to considerable variances in the reported efficacy of traditional methodologies in terms of sensitivity and specificity.
Immunoassay Tests – technological advances in the detection of ROM
The introduction of non-invasive immunoassay tests has provided clinicians with a new method of diagnosing ROM, and comparative studies with traditional methodologies have confirmed that immunoassay tests offer greater diagnostic accuracy.4 Immunoassay tests utilise lateral flo