From the Guidelines
The best targeted laboratory test to determine the likelihood of premature labor in this case is transvaginal ultrasonography (TVUS) to measure cervical length (CL). TVUS is the reference standard of imaging modalities used for assessment of the gravid cervix because it allows for complete visualization of the cervix, including the internal os where changes of preterm birth first occur 1. A short cervin (CL < 15 mm) is associated with a significant risk of spontaneous preterm birth, and TVUS has a high negative predictive value, meaning a normal CL strongly suggests that the patient will not deliver within the next 7-14 days 1.
The following points support the use of TVUS to measure CL:
- TVUS allows for complete visualization of the cervix, including the internal os where changes of preterm birth first occur 1.
- A short cervin (CL < 15 mm) is associated with a significant risk of spontaneous preterm birth 1.
- TVUS has a high negative predictive value, meaning a normal CL strongly suggests that the patient will not deliver within the next 7-14 days 1.
- The presence of cervical funneling is an independent predictor of delivery <37 weeks 1.
In contrast to other options, TVUS to measure CL is a more direct and reliable method for assessing the risk of preterm labor. Fetal fibronectin testing may also be useful, but its clinical usefulness may rest primarily with its negative predictive value given the lack of proven treatment options to prevent preterm birth 1.
Key benefits of using TVUS to measure CL include:
- High negative predictive value
- Ability to visualize the internal os
- Association with spontaneous preterm birth
- Independent prediction of delivery <37 weeks with cervical funneling
Overall, TVUS to measure CL is the best targeted laboratory test to determine the likelihood of premature labor in this case, given its high negative predictive value and ability to directly visualize the cervix 1.
From the Research
Targeted Laboratory Tests for Premature Labor
The patient in question is a 31-year-old, gravida 4, para 3 female at 33 weeks gestation with contractions. To determine the likelihood of premature labor, several targeted laboratory tests can be considered:
- Fetal fibronectin (fFN) test: This test measures the presence of fFN in cervicovaginal secretions, which can indicate an increased risk of preterm labor 2, 3, 4, 5, 6.
- Quantitative fetal fibronectin (qfFN) test: This test measures the concentration of fFN in cervicovaginal secretions, which can provide more accurate results than the qualitative fFN test 5.
- PAMG-1 test: This test measures the presence of placental alpha microglobulin-1 (PAMG-1) in cervicovaginal secretions, which can indicate an increased risk of preterm labor 5.
Comparison of Laboratory Tests
The studies suggest that the fFN test is a useful predictor of preterm labor, particularly in symptomatic women 2, 3, 4, 6. The qfFN test may provide more accurate results than the qualitative fFN test, but its clinical utility is still being evaluated 5. The PAMG-1 test has been shown to be a better predictor of spontaneous delivery within 7 days than the qfFN test, with a high negative predictive value 5.
Clinical Utility of Laboratory Tests
The fFN test has been shown to be most accurate in predicting spontaneous preterm birth within 7-10 days of testing among women with symptoms of threatened preterm birth before advanced cervical dilatation 6. The combination of fFN and cervical length measurement may provide a more accurate prediction of preterm birth than either test alone 4. The PAMG-1 test is an easy-to-use bedside test that provides rapid results and does not require specialized equipment, making it a potentially useful tool in clinical practice 5.