The reliability of chest radiographs in predicting left atrial enlargement

INTRODUCTION: Estimates of left atrial size in patients with suspected cardiac disease play an important role in predicting prognosis and events, as well as treatment decisions. Two methods are commonly used to estimate left atrial size: chest radiography and cardiac ultrasound. This study aims to determine the test characteristics by comparing the use of radiographs to cardiac ultrasound (the gold-standard test). METHODS: Data from patients older than 18 years admitted to Steve Biko Academic Hospital during 2000–2003 who had both chest radiographs and cardiac ultrasound were included in this cross-sectional, retrospective analysis. Chest radiographs were classified into three quality classes, and the sub-carinal angle (SCA) and sub-angle distance (SAD) were measured twice in all available radiographs by two observers. Intra- and inter-observer variability (three methods) as well as the predictive value of the carinal angle and sub-angle distance measurements were determined using logistic regression (with left atrial enlargement – determined by ultrasound as comparator). RESULTS: Data for 159 patients were available (154 cardiac ultrasounds and 178 chest radiographs). Intra-observer variability for chest radiograph measurements was low with almost perfect concordance (p = 0.000). Inter-observer variability was higher for supine radiographs. Using logistic regression, a linear model was identified which was statistically significant only for erect radiographs. While goodness-of-fit analysis showed that the model fits the data, performance characteristics were poor, with high sensitivity and low specificity, and an area under the ROC curve of 0.62–0.63, depending on type of radiograph and measurement (SCA or SAD). Linearity in the logit of the dependent variable was assessed, and found to be present at the extremes of SCA measurements for the supine radiograph data and in the first three quartiles for erect radiograph data. A non-linear model determined by fractional polynomial analysis did not perform significantly better than the original linear model. Cut-off values for the SCA of 72° and 84° (erect and supine radiographs, respectively) were found to give the best compromise between sensitivity and specificity. The corresponding cut-off values for SAD were 24.1 and 26.9 mm. CONCLUSION: Assessment of either SCA or SAD to determine left atrial size was equivalent and repeatable, both with the same observer and between two observers (less so for supine radiographs). While this measure was precise, it was found not to be very accurate. Therefore, chest radiographs are not reliable in predicting left atrial enlargement.