Background: Metabolic syndrome (MetS) is a constellation of metabolic conditions, including abdominal obesity, high blood pressure, high triglyceride levels, low high-density lipoprotein levels, and high fasting blood glucose levels. Adults with MetS face increased risk of chronic health conditions, such as cardiovascular disease, chronic kidney disease, depression, and sleep apnea. Various combinations of diagnostic criteria have been proposed nationally and internationally, including those by the National Cholesterol Education Program Adult Treatment Panel III (ATP III), American Association of Clinical Endocrinologists, World Health Organization (WHO), European Group for Study of Insulin Resistance, and International Diabetes Foundation (IDF). However, inconsistent use of diagnostic criteria and inadequate scientific evidence supporting the use of one combination of diagnostic criteria over another are current problems in healthcare. Objectives: We aimed to quantify the prevalence of MetS diagnoses within the patient population of our large, community-based health care system. Recognizing ATP III, WHO, and IDF definitions of MetS as possible gold standards for diagnosis, we also sought to determine the sensitivity (i.e., accuracy) achieved in documentation of MetS within our healthcare system. Methods: We conducted a retrospective review of all patients encountered within our health care system (14 hospitals and >150 clinics) during January 1, 2012-December 31, 2015. Patients were examined to determine the occurrence and associated dates of MetS diagnosis and all indications of satisfied MetS diagnostic criteria. Indications of obesity, hyperglycemia, hypertriglyceridemia, hypoalphalipoproteinemia, and hypertension, as variably defined within ATP III, WHO, and IDF guidelines, included relevant diagnoses, abnormal clinical and lab test results, and use of medications. Data describing sociodemographic characteristics were also collected. Results: In total, 1,369,620 unique patients visited our healthcare system during the study period, with 28% of patients aged ≥60 years at period start and most identifying as non-Hispanic White (76.8%) or Black (9.93%) race/ethnicity. Only 4,978 patients (0.36%) received a clinical diagnosis of MetS, despite evidence of satisfied ATP III and WHO criteria in 12.0% and 16.7% of patients, respectively. Satisfaction of IDF criteria occurred in only 0.16% of patients. With the exception of hyperglycemia, individual diagnostic criteria also demonstrated lower-than-expected rates of clinical diagnosis. For example, obesity was diagnosed in 7.86% of patients but suggested in 39.3% of patients with BMI ≥30 kg/m2; hypertriglyceridemia was diagnosed in 1.02% of patients but documented in 10.9% of patients with fasting blood triglycerides ≥150 mg/dL. The true positive rate (i.e., percentage of patients with satisfied criteria who are also clinically diagnosed) was higher in younger patients, female than male patients, and White and American Indian/Native American patients, and lowest in Black and Hispanic patients. Conclusions: Within the current healthcare system, MetS is rarely and variably diagnosed in medical practice, despite clear evidence of satisfied ATP III and WHO criteria.