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Journal of Clinical Microbiology, September 2005, p. 4613-4615, Vol. 43, No. 9
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.9.4613-4615.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Anticoagulant Carryover May Influence Clot Formation in Direct Tube Coagulase Tests from Blood Cultures

Kerry Varettas,¹ Chinmoy Mukerjee,¹ and Peter C. Taylor^1,2*

Division of Microbiology, South Eastern Area Laboratory Services, St. George Hospital, Gray Street, Kogarah, New South Wales 2217, Australia,¹ School of Medical Sciences, University of New South Wales, Kensington, New South Wales 2033, Australia²

Received 21 April 2005/ Returned for modification 23 May 2005/ Accepted 14 June 2005

The tube coagulase test (TCT) performed directly from positive blood culture bottles has been used to reduce the turnaround time for identifying Staphylococcus aureus. Most reports have shown the test to be specific but often lacking sufficient sensitivity to be useful. In a prospective study of blood culture bottles (BCB) signaling positive, with a Gram-stained smear showing gram-positive cocci resembling staphylococci, the sensitivity of the direct TCT was improved by diluting the BCB broth 1:10 in saline before inoculating 0.1 ml into 1.0 ml of 10% pooled human plasma. It was hypothesized that the improved sensitivity might be explained by reduced carryover of the anticoagulant sodium polyanetholesulfonate (SPS) used in blood culture media. By titrating the inoculum size and the concentration of SPS in an in vitro checkerboard assay, it was shown that concentrations of SPS >0.0008% prevented plasma coagulation. The 1:10 dilution of blood culture broth reduced the amount of residual SPS carried over to the TCT to a level (0.0005%) that did not impair plasma coagulation. The direct TCT inoculated with a 1:10 saline dilution of blood culture broth achieved 100% specificity and sensitivity within 4 h of inoculation without reducing the quality or quantity of coagulum.

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* Corresponding author. Mailing address: Division of Microbiology, SEALS, St. George Hospital, Gray Street, Kogarah, New South Wales 2217, Australia. Phone: (61) 2 9350 3282. Fax: (61) 2 9350 3349. E-mail: taylorp{at}sesahs.nsw.gov.au.

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Journal of Clinical Microbiology, September 2005, p. 4613-4615, Vol. 43, No. 9
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.9.4613-4615.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

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