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Spectrum Bias in the Evaluation of Diagnostic Tests: Lessons from the Rapid Dipstick Test for Urinary Tract Infection

Mark S. Lachs, MD, MPH; Irving Nachamkin, DrPH; Paul H. Edelstein, MD; Jay Goldman, MD; Alvan R. Feinstein, MD; and J. Sanford Schwartz, MD
[+] Article, Author, and Disclosure Information

Grant Support: Dr. Lachs was a Robert Wood Johnson Clinical Scholar during part of the period of this research.

Requests for Reprints: Mark S. Lachs, MD, MPH, Yale University School of Medicine, 333 Cedar Street TMP B15, New Haven, CT 06510-8025.

Current Author Addresses: Drs. Lachs and Feinstein: Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06510-8025.

Drs. Nachamkin and Edelstein: Department of Pathology and Laboratory Medicine, Hospital of the University of Pennsylvania School of Medicine, 3400 Spruce Street, Philadelphia, PA 19104-4283.

Dr. Goldman: Kaiser Foundation Hospital, Emergency Department, 27400 Hesperran Boulevard, Haywood, CA 94545.

Dr. Schwartz: The Leonard Davis Institute, University of Pennsylvania, 3641 Locust Walk, Philadelphia, PA 19104.

©1992 American College of PhysiciansAmerican College of Physicians

Ann Intern Med. 1992;117(2):135-140. doi:10.7326/0003-4819-117-2-135
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Objective: To determine if the leukocyte esterase and bacterial nitrite rapid dipstick test for urinary tract infection (UTI) is susceptible to spectrum bias (when a diagnostic test has different sensitivities or specificities in patients with different clinical manifestations of the disease for which the test is intended).

Design: Cross-sectional study.

Patients: A total of 366 consecutive adult patients in whom clinicians performed urinalysis to diagnose or exclude UTI.

Setting: An urban emergency department and walk-in clinic.

Measurements: After the patient encounter, but before dipstick test or culture was done, clinicians recorded the signs and symptoms that were the basis for suspecting UTI and for performing a urinalysis and an estimate of the probability of UTI based on the clinical evaluation. For all patients who received urinalysis, dipstick tests and culture were done in the clinical microbiology laboratory by medical technologists blinded to clinical evaluation. Sensitivity for the dipstick was calculated using a positive result in either leukocyte esterase or bacterial nitrite, or both, as the criterion for a positive dipstick, and greater than 105 CFU/mL for a positive culture.

Results: In the 107 patients with a high (>50%) prior probability of UTI, who had many characteristic UTI symptoms, the sensitivity of the test was excellent (0.92; 95% Cl, 0.82 to 0.98). In the 259 patients with a low (≤50%) prior probability of UTI, the sensitivity of the test was poor (0.56; Cl, 0.03 to 0.79).

Conclusions: The leukocyte esterase and bacterial nitrite dipstick test for UTI is susceptible to spectrum bias, which may be responsible for differences in the test's sensitivity reported in previous studies. As a more general principle, diagnostic tests may have different sensitivities or specificities in different parts of the clinical spectrum of the disease they purport to identify or exclude, but studies evaluating such tests rarely report sensitivity and specificity in subgroups defined by clinical symptoms. When diagnostic tests are evaluated, information about symptoms in the patients recruited for study should be included, and analyses should be done within appropriate clinical subgroups so that clinicians may decide if reported sensitivities and specificities are applicable to their patients.





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