Accuracy of smallpox diagnosis by immunfluorescence with a purified conjugate.

A direct fluorescent-antibody test for smallpox is described which utilizes a conjugated antivaccinia serum that was purified by diethylaminoethyl fractionation. The purity of the conjugate was analyzed by density gradient centrifugation, and specific staining of smallpox and nonsmallpox specimens was measured quantitatively by a photovoltmeter. Variola elementary bodies were identified in all of the specimens of vesicular and pustular fluid collected from 50 smallpox patients, and no false-positive diagnoses were made on specimens from 27 patients with varicella or other nonpox viral exanthems. However, 63.8% of the specimens of vesicular and pustular fluid smeared on slides in the field were unusable because of bright nonspecific fluorescence. For this reason, the fluorescent-antibody test does not compare favorably with other routine laboratory tests for smallpox.