Laboratory diagnosis of intravascular catheter associated sepsis

Many different methods have been employed to aid in the laboratory diagnosis of intravascular catheter associated infection. However, because of differences in patient populations, the definition of catheter sepsis and types of catheters, comparison of these studies is difficult. Of even more fundamental importance, the question of the pathogenesis of intravascular catheter associated sepsis (i.e. whether the microorganisms migrate to the intravascular space via the internal or external surface of the catheter) has not been resolved and is the subject of ongoing controversy. Semiquantitative culture of catheter tips would appear the easiest and most labour-efficient method available at present to diagnose catheter related infection. With central vein catheter tips, however, a cut-off level below 15 CFU per plate should be adopted as indicating a positive test result, particularly in patient populations with a high prevalence of catheter associated infection. Methods for non-quantitative broth culture of catheter tips are likely to be more sensitive than the semiquantitative method, but are less specific. Quantitative broth methods improve the specificity, but because of the labour costs involved appear not to offer much advantage over the semiquantitative method in the routine clinical laboratory. Many studies have shown that organisms are more frequently seen on staining than recovered by culture of intravascular catheters. Further studies of intravascular cathether sepsis should include a catheter staining method in addition to culture. Aspiration and culture of blood through an intravascular catheter appears to be reasonably specific in diagnosing the presence of infection on the catheter tip, but is only of low sensitivity (20-40%) in the absence of associated bacteremia.