Delayed culture of group-A streptococci: an evaluation of variables in methods of examining throat swabs.

We studied the effects on the frequency of isolation of group-A streptococci from throat swabs of school children on three varibles: (1) plating the cultures within 4 h of collecting the swabs or after storage for 4 days at room temperature; (2) using a plain or a serum-coated cotton swab; and (3) including silica gel in the swab tube or omitting this. Under the conditions of our experiment, only delayed plating of the swab gave a statistically significant advantage. When the delayed-isolation technique was used, neither the swab type nor the presence or absence of silica gel significantly influenced the result. The least advantageous combination of variables was: plating within 4 h, a plain swab, and the absence of silica gel.