The ROC curve for regularly measured longitudinal biomarkers.

The receiver operating characteristic (ROC) curve is a commonly used graphical summary of the discriminative capacity of a thresholded continuous scoring system for a binary outcome. Estimation and inference procedures for the ROC curve are well-studied in the cross-sectional setting. However, there is a paucity of research when both biomarker measurements and disease status are observed longitudinally. In a motivating example, we are interested in characterizing the value of longitudinally measured CD4 counts for predicting the presence or absence of a transient spike in HIV viral load, also time-dependent. The existing method neither appropriately characterizes the diagnostic value of observed CD4 counts nor efficiently uses status history in predicting the current spike status. We propose to jointly model the binary status as a Markov chain and the biomarkers levels, conditional on the binary status, as an autoregressive process, yielding a dynamic scoring procedure for predicting the occurrence of a spike. Based on the resulting prediction rule, we propose several natural extensions of the ROC curve to the longitudinal setting and describe procedures for statistical inference. Lastly, extensive simulations have been conducted to examine the small sample operational characteristics of the proposed methods.