Reaction difference rule for phage typing of Staphylococcus aureus at 100 times the routine test dilution.

Duplicate phage typing tests of 150 isolates of methicillin-sensitive and -resistant Staphylococcus aureus at 100 times the routine test dilution showed that the existing reaction difference rule remains satisfactory when isolates are tested on the same day; i.e., two or more reactions must differ before isolates should be considered distinct. However, if typing is done on different days, a three-reaction-difference rule is needed.