Catheter-Related Infections: It's All About Biofilm

Marcia A. Ryder, PhD, MS, RN

Disclosures

Topics in Advanced Practice Nursing eJournal. 2005;5(3) 

In This Article

Diagnosis of Catheter-Related Bloodstream Infections

Accurate and early diagnosis to validate that the catheter is the primary source of bacteremia and to identify which luminal surface is involved is essential to guide the management of catheter-related local and systemic infections.[36] Insertion site inflammation, tunnel tract infections, and port pocket infections are soft-tissue biofilm infections associated with the external surface of the catheter or port septum, and may or may not coexist with bloodstream infection.[36] CRBSIs associated with percutaneously placed nontunneled catheters that occur within the first 10 days of insertion are most often correlated with extraluminal biofilm formation.[37,38] However, more recent studies have documented the internal lumen as the primary source in short-term CVCs as early as 4 to 6 days after insertion.[39,40] Bloodstream infections related to catheters in place for longer than 10 days are almost always associated with intraluminal biofilm.[38,41,42]

The most common clinical signs and symptoms of CRBSI are inflammation or purulence at the catheter site, as well as the acute onset of fever, chills, and hypotension with no other apparent source of infection but the catheter.[43] While common, these clinical findings alone are insufficient and unreliable for a diagnosis because of their poor specificity and sensitivity.[43,44] Fever with or without chills is the most sensitive but nonspecific clinical finding associated with CRBSI. Inflammation or the presence of purulence at the catheter insertion site of nontunneled CVCs is a more specific finding, but it has poor sensitivity in the prediction of CRBSI.

In a prospective study of 1353 CVCs conducted in a university hospital, 73% of the 11 patients with CRBSI had no signs of local inflammation.[45] The sensitivity and positive predictive value of local inflammation for identification of CRBSI was 27% and 1.5%, respectively; however, purulence at insertion site -- although rare -- was found to be highly predictive of CRBSI (relative risk, 27.1; P < .005). Because of the high specificity of this finding, removal of the device is strongly recommended.

When bacteremia is present and CRBSI is suspected, the following question must be pursued: is the catheter the primary source of the micro-organisms present in the blood? Failure to obtain the appropriate cultures to address this question may lead to a false-negative or false-positive diagnosis. A false-negative diagnosis certainly may increase patient morbidity, while a false positive may result in unnecessary catheter removal or inappropriate prescription of antibiotics. This outcome is particularly dangerous because it promotes the emergence of antibiotic-resistant pathogens and generates excess costs.[42]

Disparity in the definition of CRBSI has led to highly variable practices in the clinical setting. The CDC provides 3 standard definitions in Guidelines for the Prevention of Intravascular Catheter-related Infections.[46] It is very important to use these definitions for their intended purposes. The definitions for "laboratory-confirmed BSI" and "catheter-associated BSI" are surveillance definitions that are meant to be used for benchmarking institutional performance and directing performance improvement. The clinical definition of "catheter-related BSI" presented in Table 3 is specific to the question of whether the catheter is the primary source of bacteremia. The criteria are applicable to both short- and long-term CVCs. An algorithm for selecting diagnostic tests is presented in Figure 3.

Diagnosis of catheter-related bloodstream infection.

The obvious disadvantage of using catheter cultures to diagnose CRBSI is the required removal of the device. If clinical indications warrant removal, catheter cultures in addition to catheter drawn blood cultures obtained prior to removal can facilitate treatment decisions by identifying the causative organism(s) and confirming the source of bacteremia. The CDC recommends the use of either quantitative or semiquantitative catheter segment culture techniques.

The semiquantitative roll-plate method is most commonly used because of its simplicity. A 5-cm catheter tip segment is rolled over a blood agar plate, and colony forming units are counted after incubation overnight.[35] The clinical usefulness of semiquantitative catheter cultures is questioned because it does not detect intraluminal colonization, risking a false-negative result.

Quantitative culture, accomplished by vortex, sonication, or centrifugation of the catheter segment in broth followed by serial dilution and plating on blood agar, is the most reliable catheter culture technique because it harvests most of the adherent biofilm bacteria from both the internal and external catheter surfaces.[35,44,47,48] A yield of ≥ 103 cfu/catheter segment by quantitative method, or a count of ≥ 15 cfu/catheter segment by semiquantitative method with accompanying signs of local or systemic infection indicates CRBSI.[48]

Peripheral blood cultures detect the presence of bacteria or yeast in the bloodstream but provide no information regarding the source of the micro-organisms. The CDC recommends one of 2 blood culture techniques for diagnosing CRBSI: paired quantitative blood cultures, or paired qualitative blood cultures observing a differential time to positivity (DTP). Both require the simultaneous draw of blood from a peripheral vein and from the CVC. These methods are particularly useful when retention of the catheter is desirable.

A positive result for paired quantitative blood cultures yields a ≥ 5:1 ratio of bacteria cultured from the CVC blood sample vs the peripheral sample. A single quantitative culture of blood drawn from the CVC that yields at least 100 cfu/mL is also diagnostic without a companion culture of the peripheral blood.[35,48,49] This may be of particular benefit to patients who have very limited peripheral access, or to the pediatric population where peripheral sticks are physically and emotionally problematic. Unfortunately, the use of the quantitative culture technique, although the most accurate, is limited because of its complexity, cost, and lack of methods in most hospitals to perform the test.[48,50]

An effective alternative is the paired qualitative method by differential time to positivity.[35,48,50] This technique offers accuracy comparable to quantitative blood cultures, and most hospitals do have capability to perform this comparatively inexpensive test. The DTP method monitors for bacterial growth and compares the time to positivity for both the peripheral and the CVC blood samples. A catheter-drawn blood culture that turns positive 2 hours or more before a simultaneously drawn peripheral blood culture confirms the catheter as the source of infection. The cultures should be drawn, if possible, before beginning systemic antibiotics, to avoid false-negative peripheral cultures.

When CRBSI is suspected in the presence of a multilumen catheter, each lumen should be considered a potential source of infection. Dobbins and colleagues conducted a prospective evaluation to determine the relative rates of microbial colonization in individual lumens of triple-lumen catheters and calculated the chance of detecting CRBSI if only 1 lumen was sampled.[51] They found that CVCs causing CRBSI had significant biofilm colonization in 1, 2, and 3 lumens in 10 (40%), 10 (40%), or 5 (20%) cases, respectively. Overall, the random sampling of only 1 lumen had only a 60% chance of detecting significant colonization. Therefore, sampling from each lumen is recommended to avoid false-negative results.

The decision to discard or submit the first drawn sample from the catheter that includes the heparin or saline lock solution is of frequent debate. The first drawn sample contains planktonic bacteria shed from the biofilm within the static lock solution left to dwell in the catheter. The shear effect on biofilm created by aspiration will also harvest sessile bacteria from the surface of the biofilm clusters. Discarding this sample may discard the answer being sought.

Everts and Harding assessed the benefit of discarding or flushing away the heparin lock before collecting blood for culture in tunneled and hemodialysis catheters.[52] They found no significant difference in false contamination rates between first-drawn and second-drawn samples, and no significant difference in bacterial yield or time-to-positivity results.

Guiot and colleagues assessed the value of using heparin lock fluid cultures for diagnosing CRBSI before the onset of bacteremia or clinical symptoms.[53] They found that the heparin lock fluid could detect relevant colonization of the catheter. Three or more positive heparin lock fluid cultures sampled on subsequent days correlated with the occurrence of CRBSI with a positive predictive value of 100%. To date, there is no evidence to support discarding the first-drawn sample; using it affords the benefit of reducing blood volume loss in the case of frequent blood sampling.

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