Diagnostic Tests
What Physicians Need to Know
Placental Alpha Macroglobulin-1 (PartoSure) Immunoassay to Assess the Risk of Spontaneous Preterm Birth
Am Fam Physician. 2020 Sep 1;102(5):269-270.
PartoSure is an immunoassay that measures the presence of placental alpha macroglobulin-1 (PAMG-1). PAMG-1 is a protein present in the amniotic cavity throughout pregnancy. Detection of PAMG-1 in the vagina in a patient with unruptured membranes is associated with an increased likelihood of delivery in the next seven days. PartoSure was approved by the U.S. Food and Drug Administration in 2018 for patients with a singleton pregnancy between 24 0/7 and 34 6/7 weeks' gestation, signs and symptoms of preterm labor, and cervical dilatation less than 3 cm.1 The test result is positive when at least 1 ng per mL of PAMG-1 is detected.2
| Test | Indication | Population, age range, and frequency | Cost* |
|---|---|---|---|
PartoSure (PAMG-1 immunoassay) | Assessing risk of spontaneous preterm birth | Patients with a singleton pregnancy between 24 0/7 and 34 6/7 weeks' gestation, signs and symptoms of preterm labor, and cervical dilatation less than 3 cm | $98 |
PAMG-1 = placental alpha macroglobulin-1.
*—Payment rate according to the Centers for Medicare and Medicaid Services clinical laboratory fee schedule. Accessed May 12, 2020. https://www.cms.gov/Medicare/Medicare-Fee-for-Service-Payment/ClinicalLabFeeSched/Clinical-Laboratory-Fee-Schedule-Files
| Test | Indication | Population, age range, and frequency | Cost* |
|---|---|---|---|
PartoSure (PAMG-1 immunoassay) | Assessing risk of spontaneous preterm birth | Patients with a singleton pregnancy between 24 0/7 and 34 6/7 weeks' gestation, signs and symptoms of preterm labor, and cervical dilatation less than 3 cm | $98 |
PAMG-1 = placental alpha macroglobulin-1.
*—Payment rate according to the Centers for Medicare and Medicaid Services clinical laboratory fee schedule. Accessed May 12, 2020. https://www.cms.gov/Medicare/Medicare-Fee-for-Service-Payment/ClinicalLabFeeSched/Clinical-Laboratory-Fee-Schedule-Files
Accuracy
A recent systematic review summarized test characteristics of PAMG-1 testing for predicting risk of delivery within seven days in patients with symptoms of preterm labor before 37 weeks' gestation, cervical dilatation of less than 3 cm, and clinically intact membranes.3 The review included 14 prospective or cohort studies with 2,278 total patients. All studies were published between 2015 and 2018. The trials varied in quality, and many met only one or two out of four quality criteria. The review did not list author conflicts of interest or the number of studies that were industry sponsored.
For predicting delivery within seven days, PAMG-1 testing has a sensitivity of 76%, a specificity of 97%, a positive likelihood ratio of 22.51, and a negative likelihood ratio of 0.24. The rate of delivery within seven days varied among the studies from 2.0% to 20.8%. The overall rate of delivery within seven days in the 14 studies was 6.9%.3 By comparison, the systematic review reported that for the same outcome, fetal fibronectin testing has a sensitivity of 58%, a specificity of 84%, a positive likelihood ratio of 3.63, and a negative likelihood ratio of 0.50.3 Table 1 includes posttest probabilities of delivering within seven days given varying pretest probabilities and findings on PAMG-1 or fetal fibronectin testing.3
Posttest Probabilities for a Pregnant Patient to Deliver within Seven Days Based on Pretest Probability and Results of PAMG-1 or Fetal Fibronectin Testing
| Pretest probability | Posttest probability | |||
|---|---|---|---|---|
|
| ||||
| Positive for PAMG-1 | Negative for PAMG-1 | Positive for fetal fibronectin | Negative for fetal fibronectin | |
Low risk (5%) | 54% | 1% | 16% | 3% |
Moderate risk (10%) | 71% | 3% | 29% | 5% |
High risk (20%) | 85% | 6% | 47% | 11% |
Note: Posttest probability was calculated using http://araw.mede.uic.edu/cgi-bin/testcalc.pl.
PAMG-1 = placental alpha macroglobulin-1.
Information from reference 3.
Posttest Probabilities for a Pregnant Patient to Deliver within Seven Days Based on Pretest Probability and Results of PAMG-1 or Fetal Fibronectin Testing
| Pretest probability | Posttest probability | |||
|---|---|---|---|---|
|
| ||||
| Positive for PAMG-1 | Negative for PAMG-1 | Positive for fetal fibronectin | Negative for fetal fibronectin | |
Low risk (5%) | 54% | 1% | 16% | 3% |
Moderate risk (10%) | 71% | 3% | 29% | 5% |
High risk (20%) | 85% | 6% | 47% | 11% |
Note: Posttest probability was calculated using http://araw.mede.uic.edu/cgi-bin/testcalc.pl.
PAMG-1 = placental alpha macroglobulin-1.
Information from reference 3.
Measurement of cervical length with transvaginal ultrasonography is often used to estimate the pretest probability of imminent delivery. Cervical length measurement can complement or replace PAMG-1 or fetal fibronectin testing.3–5
Benefit
The goal of predicting
References
show all references1. U.S. Food and Drug Administration. PartoSure test P-160052. Accessed July 13, 2020. https://www.fda.gov/medical-devices/recently-approved-devices/partosure-test-p160052...
2. PartoSure. Package insert. Parsagen Diagnostics, Inc; 2018. Accessed July 13, 2020. https://www.accessdata.fda.gov/cdrh_docs/pdf16/P160052C.pdf
3. Melchor JC, Khalil A, Wing D, et al. Prediction of preterm delivery in symptomatic women using PAMG-1, fetal fibronectin, and phlGFBP-1 tests: systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2018;52(4):442–451.
4. Gates M, Pillay J, Featherstone R, et al. Effectiveness and accuracy of tests for preterm delivery in symptomatic women: a systematic review. J Obstet Gynaecol Can. 2019;41(3):348–362.
5. Rundell K, Panchal B. Preterm labor: prevention and management. Am Fam Physician. 2017;95(6):366–372. Accessed July 13, 2020. https://www.aafp.org/afp/2017/0315/p366.html
6. American College of Obstetricians and Gynecologists' Committee on Practice Bulletins—Obstetrics. Practice bulletin no. 171. Management of preterm labor. Obstet Gynecol. 2016;128(4):e155–e164.
7. Berghella V, Saccone G. Fetal fibronectin testing for prevention of preterm birth in singleton pregnancies with threatened preterm labor: a systematic review and metaanalysis of randomized controlled trials. Am J Obstet Gynecol. 2016;215(4):431–438.
8. Berghella V, Saccone G. Fetal fibronectin testing for reducing the risk of preterm birth. Cochrane Database Syst Rev. 2019;(7):CD006843.
9. Wing DA, Haeri S, Silber AC, et al. Placental alpha macroglobulin-1 compared with fetal fibronectin to predict preterm delivery in symptomatic women [published correction appears in Obstet Gynecol. 2018;131(4): 745]. Obstet Gynecol. 2017;130(6):1183–1191.
10. Centers for Medicare and Medicaid Services. Clinical laboratory fee schedule. Accessed May 12, 2020.
This series is coordinated by Kenny Lin, MD, MPH, deputy editor.
A collection of Diagnostic Tests published in AFP is available at https://www.aafp.org/afp/diagnostic.
Copyright © 2020 by the American Academy of Family Physicians.
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