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Technical Briefs |
1 Department of Pathology and Immunology, Division of Laboratory Medicine, Washington University School of Medicine, 660 South Euclid Ave., Box 8118, St Louis, MO 63110
aauthor for correspondence: fax 314-362-1461, e-mail gronowski{at}pathology.wustl.edu
Numerous studies have demonstrated that fetal fibronectin (fFN) is an excellent marker of preterm delivery with a negative predictive value (NPV) >99% for predicting delivery within 7 or 14 days in symptomatic women (1)(2)(3). Interestingly, these studies have been performed by sending the sample to Adeza Biomedical for testing using an ELISA microtiter plate or by a rapid colloidal gold test marketed only outside the US. In the US, however, the only method available for rapid fFN analysis is the TLiTM system (Adeza Biomedical). Our objective was to assess the utility of the TLi rapid fFN system to predict delivery in symptomatic patients within 7, 14, or 21 days.
We used 501 cervicovaginal samples consecutively received in the Barnes-Jewish Hospital Laboratory for physician-ordered fFN analysis during an 18-month period for the study (February 2001–August 2002). From this cohort, charts for 243 patients were available for review. This constitutes the group of patients who delivered at Barnes-Jewish Hospital. Study inclusion criteria included the following: signs and symptoms of preterm labor, intact membranes, cervical dilation <3 cm, and fFN collection at 24–35 weeks. These are all criteria for fFN specimen collection included in the manufacturer’s package insert. Patients who had delivered by cesarean section or other forms of induced delivery within 21 days of fFN were excluded. If fFN measurement was performed more than once, for simplicity we arbitrarily chose the measurement made closest to delivery to be included in the study. A total of 15 patients had more than one fFN measurement performed. fFN measurements were performed according to the manufacturer’s instructions. Briefly, the patient sample was extracted from the Dacron swab into an extraction buffer (provided by the manufacturer), incubated at 37 °C in a water bath for 10 min, and filtered through a plunger filter (provided by the manufacturer). A 200-µL portion of the filtered sample was applied to the Rapid fFN cassette, and at 20 min the TLi analyzer quantified the intensity of the lines. The instrument then provided a positive or negative result. Institutional Review Board approval was obtained for this study.
Of 133 patients who met the inclusion criteria, 38 were positive for fFN and 95 were negative. The mean (SD) maternal ages at delivery for the positive and negative groups were 25.5 (5.8) and 24.1 (6.1) years, respectively (age ranges, 16.2–40.8 and 12.9–41.4 years, respectively). The mean gestational ages at the time of collection for the positive and the negative groups were 29.6 weeks (SD, 2.4; range, 24.1–33.3 weeks) and 30.0 weeks (SD, 2.8; range, 25.3–34.4 weeks), respectively. The mean gestational ages at the time of delivery for the positive and the negative groups were 35.3 weeks (SD, 3.7; range, 24.7–39.8 weeks) and 37.4 weeks (SD, 2.1; range, 30.9–40 weeks), respectively. The mean times from collection to delivery for the positive and negative groups were 39.9 days (SD, 22.5; range, 0–91 days) and 51.4 days (SD, 22.7; range, 2–107 days), respectively. Unpaired t-tests showed that only the gestational age at the time of delivery and the time from collection to delivery were significantly different between the fFN-positive and -negative groups (P = 0.003 and 0.017, respectively).
The NPV, positive predictive value (PPV), specificity, and sensitivity for predicting delivery within 7, 14, and 21 days of testing are shown in Table 1
. We also calculated the likelihood ratio for a positive result [sensitivity/(1 - specificity)]; the likelihood ratio was 2.12 for birth within 7 days, 1.92 for birth within 14 days, and 2.4 for birth within 21 days. The likelihood ratio for a negative result [(1 - sensitivity)/specificity] was 0.59 for birth within 7 days, 0.68 for birth within 14 days, and 0.53 for birth within 21 days. The data in Table 1
are similar to those obtained in previous studies using the ELISA-based assay. Joffe et al. (1) reported a NPV of 99.6% and a PPV of 9.1% for 243 patients who delivered within 7 days with gestational ages between 24 and 35 weeks of gestation. Iams et al. (2) reported a NPV of 99.3% and a PPV of 38.9% for 192 patients who delivered within 7 days with gestational ages between 24 and 34 weeks of gestation. This group also calculated a NPV of 95% and a PPV of 40% for patients delivering within 14 days. Peaceman et al. (3) examined 763 patients with gestational ages of 24–34 weeks and reported a NPV of 99.5% and a PPV of 12.7% for patients who delivered in within 7 days and a NPV of 99.2% and a PPV of 16.7% for patients who delivered in within 14 days.
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In our study, the NPV of fFN using the TLi system compared well with data from previous reports using ELISA-based assays. For patients who delivered within 7, 14, and 21 days, the NPVs were 96.8%, 93.7%, and 93.7%, respectively.
Acknowledgments
Parts of this work were presented at the AACC Annual Meeting, July 28–August 1, 2002, Orlando, FL.
References
The following articles in journals at HighWire Press have cited this article:
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  E. F. Foxman and P. Jarolim Use of the Fetal Fibronectin Test in Decisions to Admit to Hospital for Preterm Labor Clin. Chem., March 1, 2004; 50(3): 663 - 665. [Full Text] [PDF]
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