Dialysis access monitoring may help decrease thrombosis-related morbidity. We investigated the effect of time elapsed since an access flow measurement on test accuracy of a novel flow monitoring method called variable flow (VF) Doppler. A retrospective review was conducted in 36 patients with prosthetic grafts for vascular access using access thrombosis as the clinical endpoint. Receiver operator characteristic (ROC) curves and test sensitivity and specificity were determined for various follow-up time intervals. ROC analysis showed increasing test discrimination for shorter time intervals. Sensitivity and specificity for a commonly used surveillance threshold (600 ml/min) showed specificity that was little changed (88-93%) from follow-up time intervals of 15 days to 6 months. However, sensitivity was low (21%) at 6 months, increased to 50% at 2 months, 67% at 1 month, and 100% at 15 days (a single event). Low access blood flow using VF Doppler predicts near-term thrombosis. These data further imply that the discriminative value of access flow monitoring appears to be highly dependent on time from the flow measurement, improving with shorter time intervals from the measurement.