In this study, we found that the combined use of CRP and NLR may lower the cutoff of CRP in distinguishing between infectious and noninfectious inflammation in hemodialysis patients without losing the classification model’s performance (area under the receiver operating characteristics curve). Specifically, we built an algorithm using CRP and NLR for this purpose in a training population and validated it in a validation population.
The relationship between CRP and its cutoff level for early detection of active infections has been extensively studied in different populations so far (16–19). CRP levels vary greatly with age, sex and race. Additionally, in some noninfectious "metabolic inflammatory" conditions (such as smoking, uremia, cardiac ischemia) CRP levels can rise to 2–10 mg/L, while mild to moderate irritations (such as an uncomplicated skin infection, urinary tract infection or pneumonia) can raise CRP to 50–100 mg/L within six hours (16). In a prospective study of hospitalized patients aged 70 and over, it was found that the cutoff CRP value of 60 mg/L had the best combination of sensitivity (81%) and specificity (92%) for predicting bacterial infections (17). In early detecting of bacterial infection in febrile patients, CRP performance was inferior to procalcitonin with an AUC of 0.693 (0.639–0.742), a cutoff value of 73.8 mg/L, a sensitivity of 62%, and a specificity of 72%, in 326 patients admitted to the Department of Infectious Diseases in West China Hospital (18). The specificity and diagnostic accuracy of CRP were lower than that of procalcitonin also in differentiating bacterial infections from disease flare-ups in patients with systemic rheumatic diseases in a meta-analysis of eight studies, including 668 patients (19). Although the above studies investigated CRP levels as a predictor of bacterial infection, it should be realized that CRP levels cannot distinguish between types of infection, because infections in general cause CRP levels to rise, and not the type of infection (20). In dialysis patients compared to other populations, it is more complicated to interpret high levels of CRP due to a high incidence of chronic inflammation in the uremic milieu (1, 6–7). In a prospective longitudinal observational study by Snaedal et al. (21), only 13% of a cohort of 254 prevalent hemodialysis patients from six dialysis units from Sweden had constantly low CRP levels (less than 5 mg/L), whereas 19% had CRP values greater than 10 mg/L, and 68% of patients had fluctuating values depending on age, sex, comorbidity, vintage, and access type. While the diagnostic performance of serum CRP in 68 hospitalized hemodialysis patients indicating severe infections and sepsis with a CRP cutoff of 11.2mg/L, yielded a sensitivity of 89%, the specificity was only 48% (22). CRP levels above 100 mg/L were found to have a 100% positive predictive value and a 94% negative predictive value for the diagnosis of sepsis in 802 hemodialysis patients (5). Our finding of a high prevalence of infection across all CRP ranges is in line with a study by Bazeley et al. (11) exploring CRP based prediction of 1-year mortality in the DOPPS population. From our point of view, the significant presence of infection in all CRP ranges is the main reason for low Kappa scores for the inter-rater agreement observed in our study between the CRP-NLR combination predicted infection and a clinical diagnosis of infection. Of note, a CRP cutoff point of 40 mg/L that we obtained in our training population is very close to the cut-off point of 50 mg/L that is accepted in recent literature (12) for distinguishing infectious inflammation from noninfectious inflammation in chronic kidney disease population.
Integrated use of NLR and CRP in differentiating infectious and noninfectious inflammation was not done in previous studies and our study is the first of its kind. However, there have been attempts to combine CRP and NLR for other purposes, such as for predicting the prognosis in patients with gastric cancer (23), non-small-cell lung cancer (24), and in acute myocardial infarction patients undergoing percutaneous coronary intervention (25), in patients with COVID-19 pneumonia (26) and for diagnosing spontaneous bacterial peritonitis in cirrhotic patients (27). We found only the one small study performed on 100 hemodialysis patients that investigated the role of NLR, CRP and procalcitonin and their combination with a retrospective case-control design for the diagnosis of pulmonary infection (28), but without the possibility to draw clear conclusions. There are however, data in the literature of the combined use of CRP with procalcitonin, another infection marker, in dialysis patients, to differentiate between infectious and noninfectious inflammations (22, 29). The concomitant elevations in procalcitonin and CRP are hypothesized to be more sensitive in evaluating inflammation in hemodialysis patients than each marker separately (29). While procalcitonin was found as a useful marker for diagnosis of bacterial infections in hemodialysis patients with a cutoff value of 1.5 ng/ml (30), its value in making a diagnosis and predicting long-term prognosis remains doubtful in peritoneal dialysis (PD) patients with PD-related peritonitis (31). The previous study, specifying the cutoff values of both procalcitonin and CRP for early detection of infection in hemodialysis patients, was able to lower the cutoff point of CRP to 19.15 mg/L, through the combined use of procalcitonin and CRP (5). However, elevated CRP but not raised procalcitonin was found to be associated with increased inflammation and mortality in a two-year prospective study in a hospital-based cohort of high-risk hemodialysis patients (32). The combination of high procalcitonin and CRP was no more predictive of mortality than high CRP alone in this study. Still, to combine procalcitonin with CRP for the early detection of infectious inflammation, a blood procalcitonin level test is required, which is not available everywhere. In this respect, the use of NLR is much more convenient, cost-effective, available in all medical institutions and its level above 9.7 allows predicting infectious inflammation in hemodialysis patients with a CRP level above 23mg/L. Properties of CRP as an acute phase protein (33) and NLR indicate the balance between innate immune responses (neutrophils) and adaptive immune responses (lymphocytes) (34). This makes their combination a good indicator of infection and inflammation and constitute the biological basis of our study.
Our study has several limitations. First, a possible selection bias that is characteristic of the retrospective design. Second, in the absence of a gold standard for the diagnosis of infection, there may be some misclassifications of the infection status in our study. However, all similar studies use the definition of infection we used with an unavoidable methodological limitation. To overcome this limitation, all ambiguous cases were excluded from our study. Third, our study represents a single center, so the results cannot be generalized to all dialysis populations. The cutoff points of CRP and NLR should be determined to construct algorithms similar to ours in more representative populations by geographic location, races, and different ages in large epidemiological studies. Further, because of the retrospective design, we were unable to obtain several relevant markers for the diagnosis of infection such as procalcitonin.
In conclusion, we have shown that the combined use of routine laboratory tests such as NLR and CRP may be used to predict infection in maintenance hemodialysis patients, and can help in their early management to reduce the incidence of subsequent complications. Specifically, combined use of CRP with NLR may lower the CRP cutoff point in distinguishing between infectious and noninfectious inflammation in hemodialysis patients. Future large-scale studies are needed to confirm our results and apply this method to daily clinical work to improve the performance of CRP in predicting infectious diseases in maintenance hemodialysis patients.