Interleukin-6 concentrations in wound fluids rather than serological markers are useful in assessing bacterial triggers of ulcer inflammation

Abstract

Bacterial pathogenicity, microbial load and diversity are decisive for outcome and therapy of non healing ulcers. However, until now, no routine laboratory parameter is available to assess the inflammatory level caused by chronic wound infections. We thus investigated the usefulness of levels of interleukin (IL)-6 and tumour necrosis factor alpha (TNFalpha) in wound fluids for assessing ulcer inflammation in the presence or absence of microbial triggers. In addition, the predictive values of local cytokine analyses were compared with those of C-reactive protein (CRP) and lipopolysaccharide-binding protein (LBP) because serological markers are normally used to underline the suspicion of wound infections. The present data from chronic arterial and venous ulcers (n = 45) clearly show that mixed bacterial infections increased IL-6 and TNFalpha concentrations in wound fluids when compared with ulcers with a monomicrobial infection (P < 0.01 and P < 0.05, respectively). IL-6 was also significantly elevated when a high bacterial load [versus <10(5) colony-forming units (cfu)/ml: P = 0.04] or an infection with Pseudomonas was observed (versus isolation of non Pseudomonas strains: P = 0.05). Although distinct proinflammatory triggers may interfere with regard to cytokine levels, sensitivity and specificity were significant in predicting bacterial risk factors, particularly for IL-6 at a designated cut-off of 125 pg/ml (sensitivity and specificity for predicting a mixed infection: 70% and 64%, for predicting a bacterial load of >10(5) cfu/ml: 62% and 57% and for predicting an infection with Pseudomonas: 90% and 57%). In contrast to local cytokine levels, the serological markers CRP and LBP were not associated with the presence of any of the investigated bacterial triggers. Focusing on the aim of the study, IL-6 analysed in wound washouts seems to be a useful diagnostic marker for a sensitive and specific assessment of ulcers inflammation with regard to bacterial triggers.

Figure 1

IL‐6 concentrations in wound washouts seemed to reflect the level of ulcer inflammation with regard to the presence or absence of microbial triggers (data were given as mean ± SE): (A) IL‐6 was significantly increased in ulcers infected with Pseudomonas spp. (n = 12) compared with non‐Pseudomonas strains (*t‐test). However, after analyses of variance, differences in IL‐6 between the distinct bacterial families (cp‐ and cn‐Staphylococci, haemolytic Streptococci, Enterobacteriaceae and Pseudomonas spp.) did not differ significantly. (B) Furthermore, a high microbial bioburden (≥10⁵ colony‐forming units/ml; n = 28) was also significantly related to elevated IL‐6 concentrations (*P = 0·04). (C) When ulcers were classified according to the number of microbial isolates, IL‐6 and TNFα were increased (**P < 0·01 and *P < 0·05, respectively) when two or more bacterial species were isolated (polymicrobial/mixed infection: n = 22). In contrast to local cytokines, levels of CRP and LBP did not discriminate bacterial triggers of ulcer inflammation (Figure 1B and C) (In sterile ulcers, the lowest levels of cytokines were found. Because the group was small (n = 3), cases were excluded from the analyses). cn, coagulase‐negative; cp, coagulase‐positive; CRP, C‐reactive protein; IL‐6, interleukn‐6; LPS, lipopolysaccharide; TNFα, tumour necrosis factor alpha.