De todo un poco: Bench-to-bedside review: Challenges of diagnosis, care and prevention of central catheter-related bloodstream infections in children 2

Thrombosis and infection

The association between catheter infection and throm- bosis has been suggested repeatedly although the results of the older studies were never really conﬁrmed and the more recent publication is limited by size [64-66]. Although some authors argue that infection precedes thrombosis, a recent pediatric study suggested that throm- botic occlusion rather precedes infection [67]. Although ﬁbrin deposition may provoke infection [68], the vast majority of hypercoagulability disorders do not seem to predispose for CRBSI [69].

A recent Cochrane systematic review of 12 RCTs com- paring the preventive administration of anti-thrombotic agents (heparin or vitamin K antagonists) against placebo in 3,611 pediatric cancer patients only found a non- signiﬁcant trend towards decreased incidence of sympto- matic deep vein thrombosis and a nonsigniﬁcant reduction of CRBSI [70]. Another Cochrane systematic review identiﬁed two studies about preventive urokinase ﬂushing versus heparin ﬂushing, and no signiﬁcant CRBSI reduction was identiﬁed [71]. One of the included studies reported fewer occlusive events in the urokinase group but no BSI reduction [72].

In summary, the interplay between occlusion, infection and deep vein thrombosis remains unclear. Although pathophysiologically such association appears likely, no formal association has been demonstrated in children, most probably due to the fact that both events are relatively rare.

Catheter-related bloodstream infection diagnosis

Conﬁrming CRBSI in children is challenging. According to guidelines, blood cultures should be taken both from the central catheter and from a peripheral vein upon clinical symptoms, and CRBSI is most likely when the colony count from the catheter is ﬁvefold to 10-fold higher than the colony count from the peripheral vein, or when the diﬀerential time to positivity between the two blood culture samples exceeds 2 hours [73]. Peripheral sampling is painful and unpleasant, and thus is not routine practice in children. Blood culture sampling in children uses smaller volumes (1 to 3 ml compared with 10 to 20 ml in adults), and although specially enriched culture media are used, the chance of isolating live organisms drops below 70% as compared with adults [74,75]. Correctly performed blood cultures are more likely to be positive [76]. Only two-thirds of the blood samples from infants < 1 month old were adequate for culture, adequate being defi ned as containing an appropriate (age-related) volume of blood and being submitted in the correct blood culture bottle type. Th is information should prompt healthcare workers to be more careful when taking blood from neonates for blood culture [77].
If antibiotic treatment is initiated before sampling, the sensitivity of the test is further reduced [78]. Incubation times often exceeding 72 hours and low sensitivity due to blood sampling make traditional blood cultures a lengthy process to rule out BSI [79,80].

As a consequence, new identiﬁcation systems such as nucleic acid testing are promoted. Nucleic acid testing, which is based on pathogen lysis, nucleic acid extraction, puriﬁcation, ampliﬁcation and identiﬁcation, can be used either to address a single pathogen of interest or to identify a large range of bacteria and fungi, even in a simultaneous process [81]. Techniques such as nucleic acid testing may serve to identify colonies isolated by culture or they may be applied directly to blood samples [81]. Protein-based identiﬁcation via mass spectrometry by matrix-assisted laser desorption/ionization-time-of- ﬂight (MALDI-TOF) is a rapid tool for pathogen identiﬁcation, and is already widely established [82-84]. The technique uses mass spectral signals from clinical samples, which are compared with a reference database, thus allowing swift and accurate identiﬁcation. MALDI- TOF has been shown to have limitations in the identi- ﬁcation of some Gram-positive organisms or when the infection is polymicrobial [83]. However, prior culturing of organisms from clinical samples overcomes such limitation.

PCR-based diagnostics addressing housekeeping genes of microorganisms can theoretically be used directly in clinical material.

Quantitative detection of pathogen DNA without species identiﬁcation may add information in conﬁrming suspicion of CRBSI in children without prior culturing and can also detect bacterial DNA debris after initiation of antibiotic therapy [78,85]. In one recent study, there was a dose-dependent association between the amount of bacterial DNA per microliter of blood and suspicion of conﬁrmed BSI [86]. Concentrations >0.5 pg/μl had a high positive predictive value for CRBSI.

The most intensively studied clinical method of simul- taneous, pathogen-speciﬁc diagnosis is SeptiFast (Roche, Mannheim, Germany), a multipathogen probe- based real-time PCR system targeting DNA sequences of the bacterial and fungal genome simultaneously, which allows the detection of 25 common pathogens from one single blood sample [87]. The sensitivity/ speciﬁcity values (60 to 95%/74 to 99%, depending on the pathogen) and positive predictive values of SeptiFast were superior to blood cultures in a number of studies, but the diagnostic value in clinical practice still awaits the results of large, well-designed clinical trials [81,88,89]. The test is rapid, but it identiﬁes only a limited range of pathogens and does not provide information on antibiotic resistance, which is an important limitation in times of emerging resistance [87,89].

Management of catheter-related bloodstream infection

Suspected CRBSI creates the dilemma of whether or not to remove the CVC. Guidelines recommend this decision is made based on individual, clinical judgment [59,90]. The patient often still requires hemodynamic monitoring, ﬂuid intake, PEN, and medication. CVC removal would therefore result in the insertion of a new catheter with the risk of mechanical and infectious complications and would expose the child to anesthesia. There is no RCT evaluating the evidence for early versus late CVC removal in neonates [91]. Only retrospective studies with conﬂicting conclusions are published [92-95].

CRBSI must always be treated with systemic anti- biotics. However, while antibiotics are able to clear plank- tonic, free-ﬂoating microorganisms, their ability to eradicate bioﬁlm-embedded organisms is limited due to low penetration into bioﬁlm, changes in bacterial meta- bolism, antimicrobial resistance, and local alterations in the microenvironment of the bioﬁlm that impair the activity of the antimicrobial agent [96]. To eradicate microorganisms embedded in bioﬁlm, antibiotic concen- trations would have to be 100-fold to 1,000-fold that required to kill free-ﬂoating organisms [97,98]. High antibiotic concentrations instilled in the catheter lumen for hours or days alongside systemic antibiotic therapy have been used with success rates between 31 and 100% depending on the pathogen [99-106]. Most studies reported treatment durations of 14 days. Four small pediatric studies about antibiotic locks reported imme- diate success rates ranging from 83 to 100%, but no information about long-term eﬀects was provided [107- 110]. While inclusion criteria and choice of antibiotic vary greatly in these studies, the vast majority of infections in all studies are caused by CoNS.

In summary, attempts at CVC salvage are only recom- mended in uncomplicated CRBSI or CLABSI caused by bacteria that are neither too virulent nor too diﬃcult to eradicate – predominantly CoNS or enterococci [23].

Alternative strategies for CVC treatment include nonspeciﬁc disinfectants such as hydrochloric acid (HCl). HCl is thought to disrupt bioﬁlm through denaturation of protein components in the extracellular matrix, expos- ing the microorganisms to subsequent antibiotic treat- ment [111]. The instillation of 2 M HCl was described in 2004 for the ﬁrst time [112]. Three cycles of HCl exposure for 10 minutes within 1 day in combination with systemic antibiotic therapy that took into account the susceptibility of the identiﬁed pathogen resolved the infection in 28 out of 42 (67%) patients with persistent CVC-related bacter- emia, deﬁned as positive blood cultures >48 hours after the initiation of antibiotics. Another study of HCl instillation using a similar protocol was able to prolong CVC survival as compared with a historical control [111].

Although of concern, there are no data about catheter damage due to HCl treatment [113].

Ethanol also acts as a nonspeciﬁc substance interfering with the bioﬁlm matrix and has a direct disruptive eﬀect on microorganisms [114]. Attempted catheter salvage by ethanol locks with dwell times between 12 and 24 hours have been reported to be successful in 67 to 88% of published observational studies and case reports [114- 116]. A small RCT identiﬁed a reduced CRBSI incidence using a 70% ethanol lock in tunneled central lines among hematology patients [117]. A recent large RCT failed to show any signiﬁcant reduction in the incidence of endoluminal CRBSI, however, and thus its true beneﬁt remains uncertain [118]. This uncertainty is further supported by preliminary results of another large RCT including 359 long-term tunneled or implanted CVCs, which failed to show any beneﬁt of a 50% ethanol lock for CRBSI prevention [119].

Therapeutic strategies are summarized in Table 2.

Table 2. Treatment strategies for catheter-related bloodstream infections in children

		Potential					Level of
Intervention	Method	mechanism	Risk of harm	Population	Results	Comments	evidencea

Catheter removal [92-95]	Physical removal	Eliminating suspected focus of infection	Insertion of new line may be required for adequate treatment	Neonatal ICU [92,93] Children with short bowel syndrome receiving PEN [94] Children with a CVC diagnosed with candidemia [95]	Conflicting interpretations of results	Individual, clinical evaluation recommended	2b
Systemic antibiotics	Conventional treatment of bloodstream infection	Killing susceptible free-floating planktonic micro- organisms	Depends on safety profile of each drug-	–	–	No pediatric studies of catheter colonization and biofilm disruption	–
Antibiotic locks [107-110]	Luminal instillation of highly concentrated antibiotic solutions for 8 to 12 hours daily for up to 14 days	Penetrating biofilm and eradicating susceptible embedded micro- organisms	Antibiotic resistance	Pediatric patients with confirmed CRBSI [107] Adults and children receiving PEN at home with confirmed bacteremia [108] Adults and children with chronic renal failure undergoing hemodialysis with confirmed CRBSI [109] Children receiving PEN with confirmed staphylococcal CRBSI [110]	Immediate success rates between 83 and 100%	Five small studies Questionable choice of outcome Treatment occupies the line for many hours	2b
Non-antibiotic locks [111,112, 114-116]	Ethanol: luminal instillation for 12 to 24 hours	Disrupting biofilm through protein denaturation and baring micro- organisms to systemic antibiotics	Systemic side effects	Children in pediatric ICU with confirmed CLABSI [114] Pediatric oncology patients with a CVC and bacteremia [115] Pediatric patients with a CVC and persistent bacteremia (>48 hours) [116]	Ethanol: immediate success rates between 67 and 88%	Ethanol: in large trials, benefit uncertain as a preventive measure	2b
Non-antibiotic locks [111,112, 114-116]	Hydrochloric acid: luminal instillation for 3×10 minutes within 1 day		Systemic side effects Possible catheter damage	Pediatric oncology patients with a CVC and bacteremia [111,112]	Hydrochloric acid: immediate success rate = 67% (85% CI = 52.9 to 82.3%) [112] Significantly more CVCs in situ after >100 days [111]	Hydrochloric acid: very few, small studies	2b

CI, confidence interval; CLABSI, central line associated blood stream infection; CRBSI, catheter-related blood stream infection; CVC, central venous catheter; PEN, parenteral nutrition. a Level of evidence refers to Oxford Centre for Evidence-based Medicine Levels of Evidence, March 2009 [http://www.cebm.net/index.aspx?o=1025].

Discussion

Avoiding contamination upon catheter insertion and catheter care is the cornerstone of CRBSI prevention, and a number of studies support the positive eﬀect of improved routine procedures in the topic [9,15-21]. In pediatric settings, study designs are mostly quasi- experimental and neither high-quality studies nor robust analyses clearly document the eﬀect of such interven- tions. This does not imply that best-practice procedures would not work, however, and it is most likely that they contribute notably to a reduced risk of infection. Docu- mented declines in infection rates [21,120-122] allow the assumption that good clinical practice is pivotal in CRBSI prevention. These results are largely based on short-term CVCs in adult and pediatric or neonatal ICUs.

Although the contribution of the individual steps of what is called best practice is uncertain, multimodal intervention strategies were eﬀective even if the combi- nation of the speciﬁc interventions varied across the studies [123]. Bringing the risk of infection to the atten- tion of healthcare providers already positively impacts patient outcomes. This phenomenon, called the Haw- thorne eﬀect, contributes to the positive outcome of many CRBSI prevention programs and thus should always be taken into account when interpreting data from quality improvement studies – especially in open-label protocols and when using a historical control group.

Although best-practice procedures of catheter inser- tion and catheter care to prevent bacterial colonization have been proven eﬀective in CRBSI prevention, the principal pathogenesis of CRBSI is related to the catheter material itself promoting colonization with micro- organisms and the formation of bioﬁlm. As eradicating bioﬁlms is diﬃcult, if not impossible, preventing attachment and limiting bioﬁlm formation are crucial tasks alongside correct catheter care in achieving the goal of zero infection – especially with long-term indwelling devices.

Maintaining best practice may be challenging with long-term CVCs and thus prophylactic locks have been intensively studied in pediatric patients using both anti- biotics [40-42] and other antimicrobial substances [52-55] attempting to prevent the formation of bioﬁlm. Despite evidence supporting antibiotics in lock solutions [39], there has been some reluctance to recommend such locks in routine use due to growing emergence of multi- drug-resistant microorganisms [41,42,46,47]. A recent systematic review of 16 studies with a total of 176,332 CVC-days reported only one single case of bacterial resistance, but the authors argued that only few studies thoroughly addressed this issue and none performed surveillance cultures or examined long-term eﬀects (>12 months) [39]. As for catheter salvage upon sus- pected BSI, pediatric studies are limited in quantity and quality. They provide limited evidence in favor of highly concentrated antibiotic lock solutions with long in- dwelling times, although the studies vary in design and choice of antibiotic [107-110]. On the other hand, the vast majority of detected and successfully treated CRBSIs were caused by CoNS and thus the ﬁndings are in line with the advisory guidelines, which propose that an antibiotic lock may be envisaged in uncomplicated CRBSI due to CoNS and enterococci [56]. It is of interest that primary outcome was either a subsequent negative blood culture or the absence of subsequent positive blood cultures. Bearing in mind the limited reliability of blood culture sampling in children, such an outcome may over- estimate the treatment eﬀect. A recent adult RCT failed to demonstrate a treatment eﬀect because the calculated sample size was not achieved due to strict exclusion criteria [124]. Most frequently, patients were excluded because of permanent CVC use, which made an 8-hour to 12-hour lock impossible. This is frequently the case in children with CRBSI because the catheter may represent the only route of administration for ﬂuids and nutrients, and the insertion of a temporary peripheral or central line may be technically diﬃcult or undesirable due to anesthesia.

Prophylactic locks by non-antibiotic agents such as taurolidine–citrate [51], ethanol [52-54,117-119,125] and HCl [111,112] have been shown eﬀective in pediatric settings. However, such locks imply a risk of inadvertent ﬂushing or spillover to the systemic circulation. Due to low body weight this has more serious consequences in children, particularly in neonates. Taurolidine–citrate locks containing 4% sodium citrate provoked adverse events such as nausea, vomiting, and abnormal taste sensations in 20% of the children, possibly due to

spillover of citrate causing a drop in plasma concentrations of calcium and magnesium [51]. Possible adverse events at high concentrations such as 46.7%, as used in a successful trial [37], may include severe or even fatal arrhythmias due to electrolyte disturbances [126]. The side eﬀects of ethanol are dose dependent and include dizziness, lightheadedness, tiredness, headache and liver toxicity [115]. In addition, disseminated intra- vascular coagulation and thrombosis have been reported as possible adverse events in children [53,54]. Ethanol locks, like antibiotic locks, require long indwelling times and thus their use is limited given the short time potentially available for locking in children often having only a single catheter. Spillover of HCl may cause hemo- lysis [112]. Such complications have not yet been reported, but generally the possible risks of HCl remain insuﬃciently investigated.

Antimicrobial coating of CVCs is another means to prevent infection, and impregnations with antibiotics or heparin have been shown of beneﬁt [55,62,63]. However, most studies are unblinded and only one study has been performed in children. The study compared heparin coating with nonimpregnated catheters, and although CRBSI data had been obtained the reported outcome was all-cause BSI, which weakens the ﬁndings [63]. Most studies looked at catheter colonization as a primary outcome, which is questionable since a catheter may contaminate on removal. In addition, impregnated catheter tips can leak antimicrobials into the agar plate and thus provoke false negative culture results [62].

While there is evidence to support the use of antibiotic locks for CRBSI prevention, data for non-antibiotic locks in children are absent and well-designed studies are necessary to address this issue.

Quick and accurate microbiological diagnosis upon suspected CRBSI is of pivotal importance in pediatric critical care, but is challenging. New molecular tech- niques are promising, but not without shortcomings [81]. Methods such as MALDI-TOF are primarily add-ons requiring prior cultivation and although they shorten the time of pathogen identiﬁcation, they do not overcome the challenge of low sensitivity of blood cultures in children due to inconsistent blood sampling [83].

Rapid multiplex real-time PCR tests to be applied directly to clinical samples may reduce the time to diagnosis [81]. However, existing PCR-based methods have not overcome shortcomings such as limited spectra of identiﬁable pathogens, limited antibiotic resistance testing, central laboratory facility requirements, lack of 24-hour availability and cost issues [87,88]. For the time being, PCR-based methods still require conventional culture being performed because susceptibility testing is paramount in today’s emerging resistance. Ideally, PCR- based methods would be performed for rapid diagnosis of a pathogen, and blood cultures would be taken to conﬁrm pathogen identiﬁcation and to obtain antibiotic susceptibility. However, in pediatric and neonatal care settings, an additional sampling volume (for example, +1.5 ml for SeptiFast) may not be feasible due to practical limitations.

Strengths and limitations

While attempting to provide a broad review of the microbiological and clinical challenges with CVCs in children, long-term catheters appear to be over-repre- sented compared with short-term CVCs, peripherally inserted central catheters and umbilical catheters. This imbalance is partially due to the fact that short-term catheters or CVCs in the neonatal ICUs are addressed often in best-practice multimodal improvement strate- gies while technical prevention methods were more probably applied to long-term catheters.

Some discussed results are based on various adult populations [37,39,55,57,59,62]. Catheters used for adults are of diﬀerent length, diameter and material than those used for children, and there is a considerable diﬀerence in the disease spectrum indicating CVC placement and use in children and adults. There are also age-related physiological diﬀerences, which may aﬀect the risk of infections related to the devices, especially the imma- turity of the immune system and therefore relative immune incompetence in children.

Finally, children may have their CVCs longer than adults due to different clinical treatment regimes (that is, conversion from central to peripheral administration of medicine with increasing age) or merely due to the fact that critically ill children have better survival rates com- pared with adults.

Keeping in mind all of the above allows few overall evidence-based conclusions, and as such this review may leave the impression that little has been achieved. This is not the case, however, as multiple promising strategies have been proposed and indicate possible directions for future research.

Conclusion

Despite increasing research activity in CRBSI prevention in the past years, the goal of zero infection is still not in range – not for adults, and even less so for children. Insertion and care bundles addressing behavior change are diffcult to implement in many hospital settings, and technology such as locks and antimicrobial impregnation of catheters are less effec than anticipated and many of them were tested in small studies of limited quality and rarely in children. For the time being, physicians must live with the limitations of blood sampling in children upon suspicion of CRBSI. New molecular tests, although promising, must show their effectiveness and reliability in clinical practice. In general, more high- quality research is needed in the ﬁeld of infection prevention in children. This does not only apply for CRBSI prevention, but is indeed true for the entire ﬁeld of infection control.