Medicaid quality indicators track diabetes mellitus and cardiovascular disease screening in adults receiving antipsychotics and/or those with serious mental illness. To inform performance improvement interventions by evaluating the relative importance of patient, prescriber, and practice factors affecting metabolic testing. A retrospective cohort study was conducted using Missouri Medicaid administrative claims data (January 1, 2010, to December 31, 2012) linked with prescriber market data. The analysis included 9316 adults (age, 18-64 years) who were starting antipsychotic medication. Secondary analysis included the subset of adults (n = 1813) for whom prescriber knowledge, attitudes, and behavior survey data were available. Generalized estimating equations were performed to identify factors associated with failure to receive annual testing during antipsychotic treatment (adjusted odds ratio [OR], 6 encounters vs none; 0.33 [0.28-0.39]). Analysis incorporating prescriber practice information found lower failure to receive glucose testing if the patient received care at a CMHC (0.74 [0.64-0.85]) or if the initiating prescriber was a primary care practitioner (0.81 [0.66-1.00]). However, the initiating prescriber specialty-setting was not associated with lipid testing. Compared with prior reports, progress has been made to improve diabetes screening, but lipid screening remains particularly underutilized. Medicaid performance improvement initiatives should target all prescriber settings and not just behavioral health.