Rapid differentiation of methicillin-resistant Staphylococcus aureus and methicillin-susceptible Staphylococcus aureus from blood cultures by use of a direct cefoxitin disk diffusion test.

A total of 276 blood culture bottles with Staphylococcus aureus were tested by direct cefoxitin disk diffusion testing; 105 (38.1%) had zone sizes of </=17 mm (all 105 had methicillin-resistant S. aureus [MRSA]), 18 (6.5%) had zone sizes that measured 18 mm (17 had MRSA and 1 had methicillin-susceptible S. aureus [MSSA]), 8 (2.9%) had zone sizes that measured 19 mm (6 had MRSA and 2 had MSSA), 8 (2.9%) had zone sizes that measured 20 mm (6 had MRSA and 2 had MSSA), and 137 (49.6%) had zone sizes of >/=21 mm (all 137 had MSSA). Detection of MRSA/MSSA in blood cultures could be reported 10 to 24 h earlier for 88% of cultures with total accuracy.