Inoculum size and lot-to-lot variation as significant variables in the tube coagulase test for Staphylococcus aureus.

For a positive coagulase test with certain isolates of Staphylococcus aureus, inoculation of 0.5 ml of coagulase plasma with at least two colonies and 24 h of incubation at 35 to 37 degrees C may be required. S. aureus does not multiply in coagulase plasma. There is also lot-to-lot variation in the sensitivity of coagulase plasma. On isolates from critical specimens, a negative coagulase test obtained with an inoculum of less than two colonies should be confirmed by repeating the procedure with a larger inoculum or by using some other confirmatory test.