AMBIGUITY IN THE IDENTIFICATION OF STREPTOCOCCUS-PNEUMONIAE OPTOCHIN,BILE SOLUBILITY QUELLING, AND THE ACCUPROBE DNA-PROBE TESTS

We prospectively evaluated 639 sequential clinical isolates of alpha-h emolytic gram-positive cocci as possible Streptococcus pneumoniae. On the basis of results of tests for optochin susceptibility, tube bile s olubility, and the quellung reaction, 74 strains (11.6%) were categori zed as unequivocal pneumococci (optochin positive, tube bile solubilit y positive, quellung reaction positive). Among 450 optochin- and tube bile solubility-negative organisms, a subset of 56 strains was tested for quellung reaction (all negative); these isolates were categorized as unequivocal nonpneumococci. A final 115 organisms with an inconsist ent or discordant combination of susceptibility to optochin, tube bile solubility, and quellung reaction were categorized as equivocal strai ns. With the unequivocal isolates, a commercial molecular probe for S. pneumoniae (AccuProbe; Gen-Probe, San Diego, Calif) showed 100% sensi tivity (74/74) and 300% specificity (56/56). Among the 115 equivocal s trains, however, 33 (28.7%) reacted with the AccuProbe, whereas only 3 (2.6%) showed a capsule that reacted in the quellung test. A subset o f the equivocal strains identified in this group of primarily respirat ory isolates may have been S. pneumoniae that only partially expressed their classic phenotype of optochin susceptibility and bile solubilit y and only rarely expressed capsular antigens, A practical, cost-spari ng algorithm is proposed to facilitate the routine clinical identifica tion of S. pneumoniae.