Objective: To determine the usefulness of routine complete blood counts as a case-finding tool in medical outpatients.
Design: Prospective evaluation in a consecutive, unselected cohort.
Setting: A university-based outpatient clinic providing primary and referral care in general internal medicine.
Patients: A total of 595 patients (mean age [+/- SD], 40 +/- 15 years) making their initial clinic visit.
Intervention: A complete blood count was done in all patients. Components of the complete blood count that were not indicated for diagnostic or management purposes at the initial visit were classified as routine tests.
Main outcome measures: The number and nature of new diagnoses leading to new patient management; the number of additional visits and diagnostic tests precipitated by positive results.
Main results: Of the 2378 tests done, 1540 (65%) were routine tests. The percentages of abnormal test results for the four hematologic test components were as follows: leukocyte count, 7.6%; hemoglobin, 5.8%; mean corpuscular volume (MCV) of red blood cells, 7.9%; and platelet count, 4.5%. In only three patients (0.5%) was a new diagnosis made that led to a new management strategy (treatment of iron deficiency). Further work-up was unprofitable in 14 patients (2.4%). Two additional visits were necessary because of abnormal results on routine tests. No abnormality-associated clinical disease developed during a 15-month follow-up period in patients with unexplained abnormal results on routine hematologic tests.
Conclusions: The routine complete blood count has limited usefulness as a case-finding tool and has a minimal effect on care in middle-aged medical outpatients.