This AHRQ–funded study retrospectively examined the failure rate of recommended laboratory surveillance for patients on specific chronic medications. Using claims data from two large health plans, the investigators selected 11 drugs and their recommended lab testing intervals to capture and analyze data. Results suggested that nearly half of those requiring a chronic medication were subject to a potential laboratory monitoring error per year. Discussion includes detailed presentation of the potential error rates and how they differed among specific drugs and lab tests. The authors conclude that the prevalence of inadequate laboratory monitoring should stimulate further research about its impact on several clinical outcomes.