Metabolic syndrome (MetS) is a collection of several risk factors like elevated blood pressure, sugar, excessive body fat and abnormal triglyceride or cholesterol levels which collectively enhances one's risk of heart disease, stroke and diabetes. Due to its complex and heterogeneous nature, the prevalence rate of this disease is highly variable across the globe. Additionally, ethnic and regional variations affect this disparity. This paper investigates these demographic trends in MetS's diagnostic tests based on four widely used definitions, as defined by the World Health Organization (WHO), the International Diabetes Federation (IDF), the European Group for the study of Insulin Resistance (EGIR) and the National Cholesterol Education Programs Adult Treatment Panel III (NCEP-ATPIII). The study included 3581 patients (48% men and 52% women) from USA aged 12 or more. Several ethnic groups were included in the population: Mexican American, Hispanic, Non-Hispanic White and Black. Statistical measures like Student's t test, Likelihood Ratios, Receiver Operating Characteristics (ROC) curves and Youden's index were used to assess the accuracy of diagnostic tests within each ethnic group. WHO presented higher sensitivity, whereas IDF was found to exhibit least specificity. WHO was found to be a better measure for non-Hispanic black men and Mexican American women. Alternatively, NCEP-ATPIII was a better accuracy measure for Hispanic men and non-Hispanic white Americans. These results were quite stable for both genders. Results indicate that accuracy of diagnosis of MetS can be compromised by using a single consistent definition criterion over a population without considering the multi-ethnic differences.