CAP TODAY Pathology/Laboratory Medicine/Laboratory Management
Editor: Frederick L. Kiechle, MD, PhD
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November 2013
[hr] [pulledquote]Q. Can you explain the logic behind doing a full workup for identification and sensitivity on multiple positive blood culture bottles on the same patient, drawn on the same day?[/pulledquote]A. Blood cultures are considered the gold standard for detection of sepsis. While most episodes of bacteremia and/or sepsis are thought to be monomicrobial, in some instances polymicrobial sepsis, typically in immunocompromised patients, has been described.1,3 In general, two to three blood cultures should be obtained for patients suspected to have sepsis, the first two sets drawn in sequence from two different peripheral venipuncture sites. The third set should be obtained four to six hours after the initial two sets. For the majority of sepsis patients, no additional blood cultures should be obtained, and likewise obtaining a blood culture for test of cure is not recommended.1,3 Even when two to three blood culture sets are collected according to published recommendations,2,3 the length of time to detection for the individual culture bottles in each set and between sets may vary. In patients with sepsis, most blood cultures will turn positive within eight to 20 hours.
Once an index positive blood culture has been identified in a patient, a complete organism identification and antimicrobial susceptibility testing will be performed. For subsequent positive blood cultures, it is not necessary to repeat the completed and detailed organism workup that was done for the index positive culture, when the organism has the same Gram stain and colony morphology as the initial organism. In these instances, performing a few bench tests, such as catalase, coagulase, indole, PYR, and others, should be used to verify that it is the same organism/strain.2,4 The organism should be reported as “probable genus/species,” and antimicrobial susceptibility test results can be referred to the original strain. While this approach is applicable to blood cultures drawn within a 24-hour period, no guidelines have been developed for blood cultures collected past the initial 24-hour time window. Anecdotal, unpublished data suggest that polymicrobial bacteremia may be detected in such subsequent cultures, therefore making repeat full identification and antimicrobial susceptibility testing necessary.
In other instances, it might be necessary to repeat antimicrobial susceptibility testing, when the organism has developed antimicrobial resistance under treatment. Assuming that the initial two to three sets of blood cultures were obtained according to guidelines, there should be in general no need to repeat blood culture collection daily. At the earliest, five days after the first blood culture was obtained and with suspicion of treatment failure, additional blood cultures may be obtained at that point. Should those additional cultures become positive, a “repeat” full workup may be indicated. Good communication between the laboratory and the treating physician(s) is essential in these cases to optimize patient outcomes.
- Baron EJ, Weinstein MP, Dunne WM, Yagupsky P, Welch DF, Wilson DM. Cumitech 1C. Blood Cultures IV. Baron EJ, coordinating ed. Washington, DC: ASM Press; 2005.
- Garcia L. Blood cultures. In: Garcia LS, Isenberg HD, eds. Clinical Microbiology Procedures Handbook. 3rd ed. Washington, DC: ASM Press; 2010.
- Riedel S, Carroll KC. Blood cultures: key elements for best practices and future directions. J Infect Chemother. 2010; 16:301–316.
- Clinical and Laboratory Standards Institute. Abbreviated Identification of Bacteria and Yeast; Approved Guideline—Second Edition, M35-A2. Wayne, Pa.: Clinical and Laboratory Standards Institute: 2008.
Stefan Riedel, MD, PhD, D(ABMM)
Assistant Professor, Department of Pathology, The Johns Hopkins University School of Medicine
Director, Clinical Pathology Laboratories, Johns Hopkins Bayview Medical Center, Baltimore, Md.
Member, CAP Microbiology, Resource Committee
A. The basis for the initial recommendation of using a fasting sample for lipid profile testing is related to the method used at the time for measuring low density lipoprotein-cholesterol (LDL-C), the main atherogenic lipoprotein upon which most cardiovascular risk guidelines are now based.1 Until the advent of direct or homogeneous LDL-C assays about 10 to 15 years ago, almost all labs calculated LDL-C, using the Friedewald equation (LDL-C = Total Cholesterol–HDL-C–Triglyceride/5).2 This equation works because in a fasting sample, the only lipoproteins present are LDL, HDL, and VLDL. If the units are all in mg/dL, the term Triglyceride/5 provides a close approximation of VLDL-C. By using this equation, one can get a reasonably good estimation of LDL-C from a fasting sample without having to resort to a tedious ultracentrifugation step, which would otherwise be needed to separate VLDL from the other lipoproteins. HDL-C can be readily measured from a fasting sample after a precipitation step to remove LDL or VLDL.
The use of direct tests for measuring HDL-C and LDL-C, which do not require the physical separation of lipoproteins, has now made the requirement of using a fasting sample for lipid profile testing somewhat obsolete. Triglycerides can markedly increase after a meal, but for most individuals HDL-C and LDL-C do not significantly differ between the fasting and nonfasting state. Some studies suggest that for some lipid profile tests, the postprandial state may be better for predicting cardiovascular disease risk.3 Simply measuring nonHDL-C (cholesterol on all lipoproteins besides HDL) may also be better as a cardiovascular biomarker than LDL-C, and this parameter is also relatively unaffected by eating.4 Because of the difficulty in getting children to fast, the recent pediatric guidelines for cardiovascular risk testing suggest that a nonfasting nonHDL-C test is suitable as an initial screening test.5
There is concern, however, about the accuracy of some direct LDL-C and HDL-C tests, particularly in patients with dyslipidemias.6 In addition, one study has shown that a particular direct LDL-C test does not seem to be as predictive for cardiovascular disease risk when used on nonfasting versus fasting individuals.7 Because of these concerns and the extra cost of performing a direct LDL-C test, many labs still only offer the calculated LDL-C by using the Friedewald equation and thus require a fasting sample. It is, therefore, important to know what particular lipid tests a clinical laboratory is performing. The new National Cholesterol Education Program guidelines for diagnosing and treating cardiovascular disease are now also under active review, and one should await these new recommendations on the best laboratory practices for cardiovascular disease risk assessment.