Yield of laboratory tests for case-finding in the ambulatory general medical examination

To examine the diagnostic and therapeutic yield of frequently obtained laboratory tests for case-finding in the comprehensive ambulatory medical examination.

A prospective cohort study was conducted in four Mayo Clinic general internal medicine divisions that provide care to community, regional, and geographically distant patients. The main outcome measurements were the diagnostic yield and therapeutic yield of the complete blood count, chemistry panel, lipid profile, thyroid tests, and urinalysis ordered for case-finding.

Overall, 1,508 laboratory tests consisting of 7,008 individual components were obtained for case-finding in the 531 patients (mean age 63 +/- 14 years; 57% female). Thirty-six percent (544 of 1508) of the tests were abnormal, of which 6% (33 of 544) were repeated and 9% (47 of 544) led to further investigations. The 1,508 case-finding tests had a diagnostic yield of 4.8% (73 new diagnoses) and a therapeutic yield of 4.0% (60 new therapies). The therapeutic yield of each test ordered for case-finding was as follows: lipid profile (16.5%), chemistry panel (2.8%), complete blood count (0.9%), urinalysis (0.8%), and thyroid tests (0.7%). Therapeutic yield was not associated with patient's age, gender, or referral distance but was approximately twice as high in new patients compared with established patients.

The majority of treatments for conditions identified by case-finding laboratory tests resulted from the lipid profile. The therapeutic yield of the chemistry panel was low, and the therapeutic yield of the complete blood count, thyroid tests, and urinalysis were all less than 1%. The low therapeutic yield of many routine laboratory tests ordered for case-finding should be provided to patients, physicians, and managed care organizations to set priorities for case-finding and screening.