Multiple blood cultures - can you afford not to?

Contaminated blood cultures can increase hospital costs by 47% - here we learn of the benefits and cost savings of taking multiple sets of blood cultures, and the importance of drawing blood correctly

The importance of identifying and eliminating contaminated blood culture results should not be underestimated. Because failure to detect bacteraemia and toxaemia can have fatal consequences, patients are often treated prophylactically. Vancomycin is frequently used in these cases, but it has a number of drawbacks, such as autotoxicity and nephrotoxicity. This means that patients require regular monitoring to ensure that the correct blood concentration is maintained. As it can only be given intravenously, patients are required to remain in hospital, and, if it is used inappropriately, the risk of progressive antibiotic resistance increases. Detection of false positive cultures avoids inappropriate antibiotic treatment, reducing patient stays and achieving significant cost savings.

The Health Protection Agency (2010)¹ and the CLSI guidelines (2007)² recommend the collection of at least two blood culture sets per septic episode for optimal detection of bacteraemia. The collection of higher volumes of blood in multiple sets significantly increases the chances of detecting bacteria in the blood^3-5, aiding clinical interpretation of possible false positives. Collecting two or more blood culture sets, each comprising two bottles, over a twenty-four hour period will detect over 96% of bacteraemia episodes, compared to a detection rate of only 65% with the first blood culture⁴. Collecting only one blood culture set should be discouraged because it is difficult to distinguish between contamination and true septicaemia, and single blood cultures lack sufficient sensitivity for detecting bloodstream infection (BSI)⁶.

The volume of blood cultured is equally important, as a relationship has been demonstrated between contamination rate and adequate volume blood cultures^7-8. If the volume of blood collected is increased, the detection of blood stream infection improves and the patient can receive appropriate treatment. Increasing the volume also aids differentiation of true bacteraemia from contaminated samples, ensuring that MRSA bacteraemia rates are not falsely elevated and reducing inappropriate antibiotic therapy.

Contamination is a huge issue, often arising from incorrect procedure when taking blood cultures. With Trusts being monitored for MRSA bacteraemia rates, infection control nurses are becoming far more involved in educating staff to ensure that blood cultures are taken correctly. This is because MRSA from the skin can contaminate the bottles, giving rise to falsely elevated levels. Medical staff may also be offered phlebotomy training that is dedicated to blood culturing, and the use of blood culture collection packs is often advantageous.

The employment of professional blood collection teams reduces contamination rates significantly^9-12. One hospital emergency department (ED) saw contamination rates drop to an average of 3.1% when blood cultures were collected by phlebotomists, compared to 7.4% when collected by other staff. Comparison with blood cultures taken in another ED, where there were no phlebotomists, confirmed these findings, observing a contamination rate of 5.6%, with 40% of all positive blood cultures attributed to contamination. Similarly, a hospital that eliminated their phlebotomy team saw blood culture contamination rates double11, rising from 2.6 to 5.6%. Phlebotomists also have an impact on the number of blood cultures collected per patient, as they usually take multiple sets, whereas non-phlebotomists tend to draw single bottles.

Around 80% of all contaminated bottles are coagulase-negative staphylococci (CoNS), but not every CoNS will be a contaminant. BSIs usually involve only a single organism, prompting some clinicians to conclude that a blood culture that grows multiple organisms is contaminated. However, studies have shown that 6 to 21% of all true bacteraemias are polymicrobial, and usually occur in high-risk patient groups. Multiple CoNS species have also been found to cause polyclonal CoNS infections, so the presence of multiple organisms in a blood culture bottle is not conclusive evidence of contamination⁹.

One key way of distinguishing between true bacteraemia and a contaminant is to look at the number of bottles that the organisms have been isolated in^9-11,13. The enormous savings made by eliminating inappropriate antibiotic treatment and reducing the length of patient stays more than compensates for the higher initial cost of culturing extra sets. One study has shown that only 11% of patients whose positive cultures were due to contamination had multiple positive sets, in contrast to 69% of patients with true clinical bacteraemia¹³. In another study, 77% of laboratories cited the proportion of positive blood cultures in a series of cultures as very important when interpreting results¹³. When multiple cultures are obtained and return positive, the positive predictive value for true bacteraemia has been shown to improve. In a study of patients with a central vascular line and a positive culture for CoNS, the positive predictive value was 55% when one culture was positive, 20% when one of two cultures was positive and only 5% when one of three cultures was positive.  Such data makes it increasingly clear that, to maximise the diagnostic utility of blood cultures, at least two sets must be performed⁹.

The financial impact of blood culture contamination has been studied in depth in the USA, and described in a number of papers^9,11­,12. Contaminant results, compared with true negatives, have been shown to be independently associated with a 20% increase in laboratory charges and a 39% increase in intravenous (IV) antibiotic charges¹⁴. Inappropriate use of antimicrobial agents is reported to cost in excess of £3253 per patient¹⁵, and unwarranted use of vancomycin more than $4,000¹⁶ per patient. A further study focused on blood culture contamination caused by CoNS, reporting that almost half of patients with a false positive result were treated with antibiotics, often vancomycin¹². Although estimated additional costs were lower than those previously reported^15-16 (approximately £650 per patient), it was still predicted that substantially reducing unneeded therapy could save up to £130,000 per year.

Other investigators focused on the consequences of contaminated cultures in paediatric populations. An ED evaluating the additional charges associated with contaminated cultures in young children believed to be at risk of occult bacteraemia¹⁷ found costs totalled £51,300, chiefly due to subsequent hospital admission. A retrospective study¹⁸ of 9,959 blood cultures performed in children found that 26% of patients with false positive blood cultures, originally evaluated as outpatients, were subsequently admitted to hospital on the basis of their initial culture results.

False positive blood cultures have been shown to increase patient charges by 47% compared to charges for patients without laboratory evidence of bacteraemia, with the median additional hospital charge calculated at £5,674 per patient encounter¹¹. The same study also evaluated 13,800 blood cultures collected in an ED. If non-phlebotomy staff drew all the blood cultures, with contamination rates between 5.6 and 7.4%, then additional charges for patients with false positive blood cultures would range from £4.3 to £8.9 million annually. If full time phlebotomists were employed, the potential reduction in the overall ED contamination rate from 6.5 to 3.1%, could save approximately £2.6 million per year, even allowing for the annual salary of a full-time phlebotomist.

In the current economic climate, where budgets are being reduced and cost savings are essential, it is vital to consider the complete picture and not just the individual department’s budgets. The importance of collecting blood cultures using best practice and taking the optimum volume of blood is crucial. It has been clearly demonstrated that investing in staff training, using dedicated phlebotomy teams or introducing blood culture collection packs can produce significant savings, thereby justifying the additional expenditure. It is also critical to monitor which wards are producing contaminated blood cultures so that focused phlebotomy training can be given to staff working in those areas.

Major benefits can be achieved by using best practice to collect blood - taking two to three sets of blood culture bottles consisting of two bottles per septic episode will make identification of contaminated cultures easier, reduce unnecessary treatment, reduce patient stays and result in considerable overall cost savings. The message is clear; despite the higher initial outlay, maximum efficiency and lowest overall cost are achieved by using best practice to collect blood cultures.