Estimates of left atrial size in patients with suspected cardiac disease play an important role in diagnostic medicine. Left atrial size is used 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 of chest radiography and compare the use of radiographs to cardiac ultrasound (the gold standard test). Data from patients older than 18 years admitted to Pretoria Academic Hospital during 2000-2003 who had both chest X-rays and cardiac ultrasound were included in this cross-sectional, retrospective analysis. Chest X-rays were classified into three quality classes, and the sub-carinal angle (SCA) and sub-angle distance (SAD) were measured twice in all available X-rays by two observers. Intra- and interobserver variability (3 methods) as well as the predictive value of the SCA and SAD measurements were determined using logistic regression (with left atrial size determined by ultrasound as comparator). P-values < 0.05 were regarded as statistically significant for all comparisons. Data for 159 patients were available (154 cardiac ultrasounds and 178 chest radiographs). Intraobserver variability regarding chest X-ray measurements was low with almost perfect concordance (P=0.000). Interobserver variability was higher for supine X-rays. Using logistic regression, a linear model was identified which was statistically significant only for erect X-rays. 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 X-ray and measurement (SCA or SAD). Linearity in the logit of the dependent variable was assessed, and found to be present at the extremes of carinal angle measurements for the supine X-ray data and in the first three quartiles for erect X-ray 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 72o and 84o (erect and supine X-rays, respectively) were found to give the best compromise between sensitivity and specificity. The corresponding cut-off values for SAD were 24.1mm and 26.9mm. Assessment of either SCA or SAD to determine left atrial size is equivalent and repeatable, both within the same observer, and between two observers (less so for supine X-rays). While this measure is precise, it was found not to be very accurate. Therefore, chest X-rays are not reliable in predicting left atrial enlargement. / Dissertation (MSc (Clinical Epidemiology))--University of Pretoria, 2007. / Clinical Epidemiology / unrestricted