Measuring the Agreement Between two Methods to Assess Gestational Age: P060

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Introduction: In clinical practice, a new method for gestational age assessment often can only be evaluated by comparing it with other established techniques (“gold standard”). Objective: To describe, through graphical techniques and simple statistical calculations, a way of evaluating the agreement between two methods for the clinical assessment of gestational age, and thereby to determine if they are exchangeable. Material and methods: This is an observational cohort study. Between 2000 and 2001, 137 women with pregnancies of less than 16 gestational weeks were prospectively enrolled at Maternidad Sardá of Buenos Aires. The last normal menstrual period (LNMP) and ultrasound scan (US) measurements were used to the estimate of the gestational age. Mean difference (d: FUM – US), standard deviation (sd), 95% confidence interval and average value between both methods were calculated for each case and plotted (“95% limits of agreement” [d + 2 sd]). Results: Mean gestational age at first ultrasound scan was 19.6 + 4.9 (95% CI 18.7 – 20.4) weeks. Concordance between both methods increased by 71.5% (63 - 78) at +/- 10 days to 82.5% (95% CI 75 - 87) at +/- 2 weeks. The mean difference was 0.35 + 1.89 (95% CI 0.03 – 0.67) weeks and the 95% limits of agreement showed that, regardless of the gestational age, the differences reached 7.5 weeks. Conclusions: The estimate of gestational age by the last normal menstrual period was higher than that by ultrasound scan in 25% of the cases and lower in 20%, suggesting that in the clinical practice these methods are not exchangeable.