The start of a new year brought a flurry of papers on “laboratory diagnosis of sepsis”. Problems with the fundamentals of a laboratory (a geographic or abstract entity; not a person with diagnostic skills and insights) diagnosis aside, our colleagues have been busy examining the potential for molecular tests to guide early treatment of severe bacterial infection.
The core concept is to exploit the combination of speed, specificity and sensitivity of molecular biology to give a name to the biological causal agent much faster than the 12-24 hours it generally takes with culture-based methods that rely on the speed of bacterial replication. The idea is not new. It has been around pretty much as long as there have been molecular microbiologists standing by idle thermal cyclers. So what has stood in the way of progress for so long? The quick answer is the complexity of the task: septicaemia can be caused by a host of bacteria, and each requires their specific set of probes. True multiplex PCR-based methods are relatively new. The more targets, the more arduous the validation process. And the more costly the end product.
But there are other hurdles for the budding bacterial gene jockey. The first of these is that there is no true “gold standard” for septicaemia. Blood culture is prone to false positives when skin organisms contaminate the blood inoculated into culture vials, explaining why so many contain coagulase negative staphylococci, Corynebacterium and Propionibacterium species. There are also factors that reduce sensitivity, for example, variable but generally low quantities of blood use to inoculate blood culture bottles, variable showering of bacterial into the peripheral circulation, and prior use of antibiotics that may have already begun to inhibit or kill the very bacteria you’re trying to recover in the lab. Once we start using molecular methods, we stray into tiger country with potential inhibition by human DNA, or the anticoagulants used to stop the blood in the culture bottle from clotting. A tough job then.
So I applaud the huge effort the groups that have published have put in, and I welcome their optimism that we can do better. But as we read their reports, we need to ask the hard questions about how close their method is to offering a point-of-care support to the physicians who make those initial clinical decisions.
It strikes me that there are several key questions that any study of this kind needs to answer:
- Can the method offer a validated, definitive result less that four hours after the patients arrives at the hospital?
- Exactly how sensitive is it (i.e. how much blood do you need from a patient with septic shock to guarantee a positive result)?
- How wide a range of organism-specific targets are on offer and what percentage of septicæmias will this cover at the test site?
- Can the test procedure operate a round-the-clock service?
- How do the designers propose to integrate the method into both clinical laboratory practice and front line clinical medicine?
Two recent review articles provide a useful entry point to this complex area. They can be summarized by the current verdict: “promising technology; needs to try harder”.
- Mancini N, Carletti S, Ghidoli N, Cichero P, Burioni R, Clementi M. The era of molecular and other non-culture-based methods in diagnosis of sepsis. Clin Microbiol Rev. 2010 Jan;23(1):235-51.
- Dark PM, Dean P, Warhurst G.. Bench -to-bedside review: the promise of rapid infection diagnosis during sepsis using polymerase chain reaction-based pathogen detection. Crit Care. 2009;13(4):217. Epub 2009 Jul 15.
- Avolio M, Diamante P, Zamparo S, Modolo ML, Grosso S, Zigante P, Tosoni N, De Rosa R, Stano P, Camporese A. Molecular identification of bloodstream pathogens in patients presenting to the emergency room with suspected sepsis. Shock. 2010 Jan 19. [Epub ahead of print]
- Bloos F, Hinder F, Becker K, Sachse S, Dessap AM, Straube E, Cattoir V, Brun-Buisson C, Reinhart K, Peters G, Bauer M. A multicenter trial to compare blood culture with polymerase chain reaction in severe human sepsis. Intensive Care Med. 2010 Feb;36(2):241-7. Epub 2009 Nov 19.
- Lehmann LE, Hunfeld KP, Steinbrucker M, Brade V, Book M, Seifert H, Bingold T, Hoeft A, Wissing H, Stüber F. Improved detection of blood stream pathogens by real-time PCR in severe sepsis. Intensive Care Med. 2010 Jan;36(1):49-56. Epub 2009 Sep 15.
- Tissari P, Zumla A, Tarkka E, Mero S, Savolainen L, Vaara M, Aittakorpi A, Laakso S, Lindfors M, Piiparinen H, Mäki M, Carder C, Huggett J, Gant V. Accurate and rapid identification of bacterial species from positive blood cultures with a DNA-based microarray platform: an observational study. Lancet. 2010 Jan 16;375(9710):224-230. Epub 2009 Dec 10.
- Tsalik EL, Jones D, Nicholson B, Waring L, Liesenfeld O, Park LP, Glickman SW, Caram LB, Langley RJ, van Velkinburgh JC, Cairns CB, Rivers EP, Otero RM, Kingsmore SF, Lalani T, Fowler VG, Woods CW. Multiplex PCR to diagnose bloodstream infections in patients admitted from the emergency department with sepsis. J Clin Microbiol. 2010 Jan;48(1):26-33. Epub 2009 Oct 21.
µGnome, 31st January, 2010