This study demonstrates that the OPNI is associated with the nutrition status and comorbidities. This index was less correlated to inflammation or the edema index than the serum albumin. Univariate and multivariate analyses showed that the initial low OPNI was associated with mortality in PD patients.
This study shows that the edema index and CRP are negatively correlated with serum albumin. Serum albumin has been well documented as a risk factor for mortality in dialysis patients (1
). Canada-USA Peritoneal Dialysis Study Group (CANUSA) results showed that 1 g/L increase in serum albumin is associated with 6% decrease in mortality (16
). However, serum albumin in patients with comorbidities may be more associated with inflammation or hydration than the nutritional status. While hypoalbuminemia from malnutrition alone is uncommon, malnutrition combined with hydration status or inflammation play a role in causing hypoalbuminemia in dialysis patients (17
). Therefore, serum albumin in patients during the early dialysis period may be less effective as a nutritional marker than that in stable dialysis patients. Multivariate analysis adjusted for the edema index shows that serum albumin is not associated with mortality in PD patients. We found that that the correlation coefficient of OPNI for the CRP and edema index was lower than that in serum albumin. Additionally, OPNI was associated with mortality in PD patients after multivariate adjustment for the edema index and CRP. OPNI may be an independent prognostic factor, regardless of inflammation and hydration status.
Total lymphocyte count has been proposed as a prognostic factor (18
). Increased total white blood cell count has been found to correlate with mortality in elderly men or after myocardial infarction (19
). Increased neutrophil counts have been implicated as a biomarker of atherosclerosis and/or inflammation (21
). Malnutrition can induce a decrease in total lymphocyte count and suppression of cellular immunity including delayed hypersensitivity reaction (22
). Immunologic changes occur early in the course of nutritional depletion (18
). Reddan et al. (23
) showed that the lymphocyte count was associated with the nutritional status and mortality in dialysis patients. This study shows that total lymphocyte count is an independent marker for the edema index or CRP. Although there was no statistical significance, low total lymphocyte tertile results in a hazard ratio of 1.337 for mortality. In our study, a total of 27 patients had liver or spleen diseases, which were associated with changes in the total lymphocyte count. The number of patients with 2 conditions was low, and the mortality risk of OPNI after adjusting the comorbidity index, including liver disease, was significant. These show that the clinical significance for OPNI may be valuable after correcting for these effects.
This study shows that a low OPNI score is associated with mortality in PD patients. This variable is less correlated to the edema index and CRP than serum albumin. This may be associated with addition of total lymphocyte count as an independent factor for edema index and CRP within the OPNI equation. The association between the OPNI, serum albumin, and total lymphocyte count may be due to inclusion of two variables within the OPNI equation. However, low OPNI was also associated with old age, high grade of the Davies risk index, low serum creatinine, and fat mass.
An ideal nutritional marker should be simple, have association with the nutritional status, and predict mortality. Many nutritional indices have been introduced in dialysis patients (6
). While these methods have been validated, using the OPNI is a simpler and easier method than previously reported indices. This index only includes complete blood count with differential count and serum albumin. This does not require additional laboratory findings or subjective questionnaires. This method has been validated in patients with malignancy or postoperative patients, who are expected to have the malnutrition in combination with inflammation, edema, or comorbidities (8
). This study shows that the OPNI can be clinically relevant in PD patients, who are expected to have the combination of inflammation or edema.
Validation studies for OPNI were limited to Japanese subjects. There have been 4 reports examining the prognosis in patients with gastrointestinal malignancy, active tuberculosis, or end-stage liver disease (8
). There has been a report regarding the association with nutritional status alone and two reports regarding postoperative complications in patients with gastrointestinal malignancy or active tuberculosis (9
). OPNI validation has not been widely studied in chronic kidney disease patients. This may be due to several limitations such as the application of widely known serum albumin levels and total lymphocyte count as variables for OPNI and changes in OPNI based on variable conditions. Moreover, it is not known if the results can be applicable to other populations.
There are a few limitations to this study. This was a retrospective, single-center study. Although measurements using OPNI calculation were measured 1 or 2 months after the initiating PD and patients had no definite signs of infection, OPNI was calculated using a single measurement of the total lymphocyte count. A time-averaged value may be a more precise method for predicting mortality than a single value alone. Additionally, we were unable to evaluate the lymphocyte subset. Malnutrition is associated with a decreased CD4:CD8 ratio as well as the appearance of peripheral immature T cells (24
). Evaluating the lymphocyte subset will help to differentiate the independent effect of malnutrition from variable conditions associated with lymphopenia, such as sepsis, malignancy, and steroid treatment (18
). Decreased lymphocyte type is typically associated with a specific nutrient, e.g., the known association between β-carotene and CD19 (26
). Understanding these relationships may be valuable for identifying the insufficient nutrient. Further investigation involving a more accurate lymphocyte count or subset is necessary.
In summary, our results demonstrate that the OPNI is less related to inflammation and edema than serum albumin per se. An initial low OPNI is associated with poor nutritional status and high mortality in PD patients. The OPNI is a simple method that can be used for predicting the nutritional status and clinical outcome in PD patients.