Fetal fibronectin test: what it measures and how it is used in suspected preterm labour

Fetal fibronectin is a protein found where the amniotic sac attaches to the womb. Tommy's, the pregnancy charity whose Edinburgh centre ran UK research on the test, describes it as a special protein made by the baby's cells that acts as a glue. When birth is imminent, the protein leaks out into the vagina and can be detected by taking a swab.

The test therefore looks for a sign that the body may be getting ready to deliver. A swab is taken from the vagina, usually at a speculum examination, and analysed. Tommy's says that results can be given in less than ten minutes, and that a test giving a simple positive or negative answer has been available for a while. Newer analysers can measure the exact concentration, and the lower the concentration, the less likely it is that a woman will give birth early.

The test is used only for people with symptoms, such as contractions or abdominal pain, who are being assessed for possible preterm labour. It is not a routine screening test in a symptom-free pregnancy.

Many people who attend a maternity unit with signs of early labour do not go on to deliver soon. Tommy's says that up to 80% of women who have signs of premature labour, like contractions or stomach pain, are still pregnant after 7 days, but it is very difficult to tell which women are actually at risk of premature birth. As a result, a lot of women are treated just in case, because doing nothing could have devastating consequences.

A test that reliably identifies who is unlikely to deliver soon allows clinicians and families to avoid some admissions, injections and transfers. A test that reports a higher chance supports giving treatment promptly. The concept is not new, but its value depends on how it is used and which results are acted upon.

The NICE guideline on preterm labour and birth (NG25) sets out a stepwise approach for women with symptoms of preterm labour and intact membranes. First, a clinical assessment should be offered, including history taking, observations and a speculum examination. NICE adds that if a swab for fetal fibronectin testing is anticipated, it should be taken before any digital vaginal examination.

If the assessment suggests suspected preterm labour at 29+6 weeks or less, NICE advises treatment for preterm labour as described in its sections on tocolysis and maternal corticosteroids. From 30+0 weeks, NICE says to consider transvaginal ultrasound measurement of cervical length as a diagnostic test to determine the likelihood of birth within 48 hours. Only if cervical length measurement is indicated but not available or not acceptable should fetal fibronectin testing be considered.

This means the fFN test is generally the second choice behind cervical length, and it is not used for very early gestations, where treatment is advised based on the clinical assessment. NICE also says clinicians should explain the tests to the woman, including the consequences of false-positive and false-negative results and taking gestational age into account.

NICE sets the threshold at 50 ng/ml. If the test is negative, with a concentration of 50 ng/ml or less, the guideline says to explain that it is unlikely she is in preterm labour. The clinician should think about alternative diagnoses and discuss the benefits and risks of going home compared with continued monitoring and treatment in hospital. If she decides to go home, she should be advised to return if symptoms suggestive of preterm labour persist or recur.

If the test is positive, with a concentration of more than 50 ng/ml, NICE says to view the woman as being in diagnosed preterm labour and to offer treatment as described in its sections on tocolysis and maternal corticosteroids. Tocolysis refers to medicines that try to slow or stop contractions, and corticosteroids are given to help the baby's lungs mature.

If a woman at 30+0 weeks or more does not have either cervical length measurement or fFN testing to exclude preterm labour, NICE advises offering treatment consistent with diagnosed preterm labour. This is why the test can make a practical difference: a negative result can allow a safe, supported return home, whereas without a test, the default is to treat.

Other swab tests detect different substances in cervical secretions. NICE's HealthTech guidance HTG476, published in July 2018, assessed three biomarker tests: Actim Partus, PartoSure and the Rapid fetal fibronectin 10Q Cassette Kit at thresholds other than 50 ng/ml. NICE said there was not enough evidence to recommend that these tests can be used routinely in the NHS, and that further research is needed on their accuracy and on how they affect treatment for the mother and baby.

NICE added that qualitative fetal fibronectin testing at a threshold of 50 ng/ml is already recommended in its guideline on preterm labour and birth and is not affected by that guidance. Local hospitals choose which tests they stock, so the test you are offered may differ from the one described here.

Tommy's describes the QUIDS study, funded by the National Institute for Health Research and run from 2015 to 2020 through its Edinburgh centre with clinicians across the UK. The researchers looked at whether the quantitative fFN test could be combined with other warning signs, such as a history of previous preterm labour or a twin pregnancy, to predict who would give birth within 7 days of testing.

The resulting model was trialled in women with symptoms of preterm labour who attended 26 UK maternity units, and Tommy's reports that the model was excellent at predicting who would go into labour spontaneously in the next 7 days. The charity's hope was that the right women would get the support they need while others avoid unnecessary treatment. This is research, and local practice follows NICE guidance and trust protocols.