Background: Biomarker measurements improve the phenotyping of patients with severe uncontrolled asthma (SUA) and predict therapeutic responses. The use of biomarkers in asthma, however, remains underused. Objective: To test the hypothesis that biomarker measurements of patients with SUA remain markedly underused and contributes to asthma morbidity and oral corticosteroid use. Methods: Leveraging claims data linked to electronic health record data, we calculated biomarker use by providers treating patients with SUA from January 2017 to August 2020. Results: From 3.6 million clients, 3817 had a primary diagnosis of asthma; most were between 50 and 60 years old. Also, 63.2% were female patients; those under ages 10 years were primarily boys. Of the 728 patients who reported race, 69.9% were white and 21.8% were African American. Of the 840 who reported ethnicity, 14% were Latinx. A predetermined definition of SUA identified 348 patients with SUA. In a nested sample of 151 patients with SUA, 43% were managed by primary care physicians (PCP), 4% by specialists, and 49.7% by both. Of this sample, 61.5% had a measurement of serum eosinophils, 9.9% total immunoglobulin E values, and 9.3% radioallergosorbent skin tests; 38% received no tests, whereas 9.9% had more than one. Specialists ordered a biomarker test 4.6 times more often than did PCPs, whereas PCPs ordered 70% of the prednisone prescriptions for recurrent asthma exacerbations. Conclusion: Specialists were more likely to order biomarkers than were PCPs. Patients managed exclusively by PCPs were more likely prescribed oral prednisone. Real-world evidence shows that biomarkers are infrequently used to characterize patients with SUA, especially among patients exclusively managed by PCPs. Programs that encouraged biomarker use may improve SUA management and oral corticosteroid burden.