Introduction

Red cell distribution width (RDW) is routinely examined with the complete blood count (CBC) test that shows the heterogeneity in erythrocyte size. It is used clinically to differentiate different types of anemia. Recent studies have reported the association between high RDW levels and increased mortality in the general population [1,2], in patients with cardiovascular disease [3,4], brain vascular disease [5], septicemia [6], chronic obstructive pulmonary disease [7], and hepatitis B [8]. Mortality, the activity of inflammatory bowel disease [9], and pulmonary functions [10] have also been reported to be associated with RDW values. The mechanism underlying those associations of RDW with survival or disease activity has not been elucidated, but high levels of RDW are thought to be provoked by chronic inflammation, poor nutritional status (e.g. iron, folate, and vitamin B12 deficiency), and age-associated diseases via changes in erythropoiesis [1,11,12].

Malignant tumors are known to evoke chronic inflammation and malnutrition [13]. The resulting low performance status (PS) and compromised nutritional status account for the patient’s poor quality of life, and are obstacles in cancer treatment. Generally, markers such as age, PS and disease stage are used to risk-stratify patients and guide therapeutic strategy. These parameters provide some useful information, but the clinical conditions of cancer patients are so complex that finding biomarkers to define the patient’s general condition remains a significant challenge.

We hypothesized that RDW levels might be associated with cancer progression because RDW reflects chronic inflammation and nutritional status. There are few reports on the relationship between RDW and malignant tumors [14-19], and they are limited to small studies that distinguish malignant from benign tumors, or predict a malignant tumor. In particular, there has been no specific study on the relationship of RDW with cancer stages or patient survival. Therefore, in this study, we investigated RDW levels in lung cancer patients and determined the relationships between RDW and various clinical parameters, including survival in lung cancer patients.

Methods

Results

RDW values and cancer stages

Patient RDW values according to cancer stages were examined, and we found an association between cancer stage and RDW (p<0.0001, Kruskal-Wallis test, Figure 1A). The RDW values were also associated with cancer stage in patients without any other diseases (p<0.0001, Kruskal-Wallis test, Figure 1B).

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Figure 1. RDW levels in lung cancer patients according to stage.

(A) RDW levels of lung cancer patients. Kruskal-Wallis test: p<0.0001. Tukey non-parametric test: p-value I–II=0.58; I–III=0.039; I–IV<0.001; II–III=0.29; II–IV=0.021; III–IV=0.13. (B) RDW levels of patients with lung cancer without any comorbid disease. Kruskal-Wallis test: p<0.0001. Ad hoc, Tukey non-parametric test: p-value I–II=0.96; I–III=0.043; I–IV=<0.0001; II–III=0.13; II–IV<0.0001; III–IV<0.001. The box plots in the figure represent columns of data as boxes whose extents indicate the 25^th and 75^th percentiles. The line inside the box represents the median. Capped bars indicate the minimum and maximum values.

https://doi.org/10.1371/journal.pone.0080240.g001

RDW values and prognosis

We performed a prognostic study of 332 patients followed up for 24 months. Within the group, 125 patients died from lung cancer-related causes. The survival rate is shown in Figure 2. As shown in Figure 2A, groups with all stages of lung cancer having high RDW values had a worse prognosis than those with low RDW values (Wilcoxon test: p=0.002).

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Figure 2. Survival rates of lung cancer patients stratified by RDW.

(A) Survival rates for patients with stages I-IV lung cancer (n=332, high RDW (n=73) vs low RDW (n=259)). (B) Survival rate for patients with stages I and II lung cancer (n=141, high RDW (n=19) vs. low RDW (n=122)). (C) Survival rate for patients with stages III and IV lung cancer (n=191, high RDW (n=54) vs. low RDW (n=137)).

https://doi.org/10.1371/journal.pone.0080240.g002

Next, we divided 332 patients into two groups: early stages of lung cancer (stages I and II, n=141, Figure 2B) and progressive stages of lung cancer (stages III and IV, n=191, Figure 2C). As shown Figure 2B, among the patients with early stages of lung cancer, the patients with high RDW values had a worse prognosis (Wilcoxon test: p<0.001). Meanwhile, no statistically significant differences were observed between the two groups in patients with progressed lung cancer stages (Figure 2C, Wilcoxon test; p=0.27).

The results of the Cox regression analysis are presented in Table 4. The group with higher RDW values showed significant differences (hazard ratio=2.15, 95% confidence interval 1.04-4.46, p=0.040) when we controlled for stage, PS, presence of other disease, presence of treatment, albumin, CRP, and interaction of stage and RDW. We also identified borderline significant interaction between RDW values and cancer stage (p=0.062), which was consistent with subgroup analysis based on cancer stage (early stages and progressive stages).

Variables	Univariate analysis		Multivariate analysis
	HR (95% CI)	p-value	HR (95% CI)	p-value
RDW ≧15 vs <15 (%)	1.33 (1.10-1.59)	0.003	2.15 (1.04-4.46)	0.040
Stage		<0.0001		<0.0001
II vs I	2.80 (1.13-7.05)		2.18 (0.70-6.12)
III vs II	3.63 (1.90-7.67)		2.82 (1.29-7.75)
IV vs III	1.57 (1.09-2.29)		1.39 (0.88-2.22)
ECOG PS		<0.0001		<0.001
1 vs 0	1.81 (1.14-3.01)		1.88 (1.16-3.14)
2 vs 1	2.63 (1.72-3.96)		1.82 (1.16-2.81)
3 vs 2	1.68 (0.89-3.04)		0.67 (0.34-1.25)
4 vs 3	4.05 (0.93-12.6)		3.91 (0.84-13.4)
Other diseases y vs n	1.19 (0.85-1.66)	0.31	0.94 (0.78-1.14)	0.55
Treatment y vs n	0.43 (0.35-0.53)	<0.0001	0.53 (0.43-0.67)	<0.0001
Albumin	0.27 (0.19-0.38)	<0.0001	0.54 (0.33-0.90)	0.019
CRP	1.14 (1.11-1.18)	<0.0001	1.05 (1.00-1.10)	0.033
Stage*RDW		0.001		0.062

Table 4. Univariate and Multivariate Cox proportional hazard model for survival (n=332).

HR, hazards ratio; CI, confidence interval; RDW, red cell distribution width; ECOG PS, Eastern Cooperative Oncology Group Performance Status; CRP, C-reactive protein; y, yes; n, no.

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Discussion

To our knowledge, the present study is the first to analyze RDW in lung cancer patients. We revealed two major findings: (1) RDW values were positively associated with clinical cancer stage; and (2) patients with higher RDW values had poorer prognoses.

As shown in Tables 2 and 3, advanced stage, poorer PS, high WBC, lower albumin, and higher CRP were associated with high RDW. This result might support the idea that high levels of RDW reflect chronic inflammation and poor nutritional status of patients with lung cancer. In addition, the presence of comorbid diseases, which might provoke inflammation and poor nutritional status, was also associated with RDW values (Table 2), implying that comorbidity might also contribute to RDW values. This result is in accordance with previous reports that showed an association of high RDW values and some diseases [3-10]. Next, as shown in Figure 1, the results of Kruskal-Wallis tests revealed a positive association between clinical cancer stage and the level of RDW (Figure 1A, B). This result might also reflect an association between RDW and increased inflammation or malnutrition induced by cancer progression.

Next, we revealed a significant association between high RDW and poor prognosis of cases within any stage of lung cancer (Figure 2A). Because RDW values are positively associated with cancer stage as shown in Figure 1, it is not surprising that univariate analysis performed on patients in any stage showed that patients with higher RDW values had shorter survival. Therefore, we divided cancer stages into two groups: the early stage group (stage I, II) and progressed stage group (stage III, IV), to exclude the stage effect on survival rate (Figure 2B, C). When cancer stage was aligned, the group with higher RDW values had a significantly poorer prognosis in the early-stage group. Naturally, there are differences in prognosis in each case even if they are in the same cancer stage, because many confounding factors are involved in survival. One explanation for the difference in prognosis in early-staged patients might be the strong association of RDW value with PS, albumin, and CRP. Poorer survival in patients with poor PS, low levels of albumin, and higher levels of CRP is well established [22-26], and our study indicated similar results (Table 4), showing that they are independent prognostic factors. In addition to these factors, RDW was also an independent prognostic factor. One merit of RDW assessment is that it can be described without any human bias, whereas PS scores rely on a clinician’s perception of a patient’s condition.

Inflammation also impacts patient survival [25]. The exact mechanisms of inflammation on RDW levels are unknown, but potential mechanisms include impairing iron metabolism, inhibiting the response to erythropoietin, and decreasing red blood cell survival via production of inflammatory markers [27,28]. The overproduction of selective cytokines such as interleukin-6, tumor necrosis factor α, and CRP has been shown to play a key role in inducing chronic inflammation in cancer patients [27]. In addition, chronic inflammation is also reported to lead to a poor response to chemotherapy [22]. Hence the poorer survival in patients with higher RDW values might be due to chronic inflammation itself, or the poor response to chemotherapy; however, further investigation is needed to explain the relationships of RDW with inflammation and the response to cancer treatment.

The study subjects in the prognostic analysis included those with all types of histology, and the presence or absence of cancer therapy was not distinguished. Generally, patients with the initial stage of non-small cell lung carcinoma (NSCLC) are best treated with surgery when possible. However, palliative chemotherapy, radiotherapy or even best supportive care is sometimes chosen depending on the patient’s general condition. Decision-making in cancer treatment strongly relies on patient status and complex factors, such as nutritional status, PS, and the presence of comorbid disease. Since the cases with high RDW values reflected each patient’s poorer general condition, best supportive care instead of radical operation or chemotherapy might tend to be chosen for them. Meanwhile, there was no significant difference in prognosis in patients with stage III and IV lung cancer, indicating that many other factors not directly reflected by RDW values play a greater role in survival when cancer has progressed.

There are some limitations in this study. For instance, the relationships between RDW and histological types were not made clear in this study. Lung cancer is classified into two major histological types, small cell lung carcinomas and NSCLC, and the clinical course and response to treatment greatly vary depending on the histology. There was no differential association of RDW levels across lung cancer histology (not shown). Since the number of the each histological type was small in this study, further investigation is needed to determine the associations of RDW and lung cancer histology.

Because we considered a large number of factors in analyses, multiple comparisons are necessary in a confirmatory study. However, our study was an exploratory one to search for any factors associated with RDW values. If adjustments for multiple comparisons were made in exploratory studies, important associated factors would be missed. Because adjustments for multiple comparisons were not made in our study (as in a number of previous exploratory studies) it will be necessary to validate factors identified in our study in future confirmatory studies.

In conclusion, this study revealed an association of RDW values with PS, albumin levels, WBC counts, platelet counts, the presence of comorbid disease and clinical cancer stage. Moreover, significant associations between higher RDW levels and poor survival, especially in patients with early stage lung cancer, were also shown. In cancer treatment, as patients’ clinical courses vary and are difficult to predict, stratification of patients for appropriate surveillance programs and different tailored therapeutic strategies is important. Because RDW values can be routinely examined by CBC tests, RDW might be a new and convenient marker to understand the patient’s general condition and to predict the mortality risk of a patient within a determined lung cancer stage.