Clumping in both drops is an indication of autoagglutination, so a tube test should be carried out. Tube test is not performed each institutions but most of the result depends on blood cultures from lab. Tube test The tube test uses rabbit plasma that has been inoculated with a staphylococcal colony (i.e., Gram-positive cocci which are catalase positive). The tube is then incubated at 37 °C for 1.5 hours. If negative, then incubation is continued up to 18 hours. If 'positive' (e.g., the suspect colony is S. aureus), the plasma will coagulate, resulting in a clot (sometimes the clot is so pronounced, the liquid will completely solidify). If 'negative', the plasma remains a liquid. The negative result may be S. epidermidis but only a more detailed identification test can confirm this, using biochemical tests as in analytical profile index tests methods. A false negative can be perceived if the sample is not allowed to cool for about 30 minutes at room temperature or 10 minutes in the freezer, given that the serum can melt. If truly negative, the serum will remain liquid after cooling. List of coagulase-positive staphylococci: Staphylococcus aureus subsp. anaerobius, S. aureus subsp. aureus, S. delphini, S. hyicus, S. intermedius, S. lutrae, and Staphylococcus schleiferi subsp. coagulans. List of coagulase-negative staphylococci of clinical significance: S. saprophyticus, S.cohnii subsp. cohnii, S. cohnii subsp. urealyticum, S. captitus subsp. captitus, S. warneri, S.hominis, S.epidermidis, S. caprae, and S.lugdunensis References External links Tube coagulase test - rabbit plasma video Coagulase test procedure Microbiology EC 3.4.23