Red blood cell distribution width (RDW) is an index depicting the size variation of red blood cell (1). For a long time, RDW has been used to explore the causes of anemia. In 2007, Felker et al. (2) reported that RDW is a promising prognostic marker in patients with heart failure. Subsequently, accumulated studies have revealed that RDW is a diagnostic marker, risk factor and prognostic factor in various cardiovascular diseases (3). Although the association between RDW and cardiovascular diseases has been revealed in many studies, the mechanism underlying the association is largely unknown. In 2009, Lippi et al. (4) reported that RDW is positively correlated with inflammatory markers [e.g., C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR)] in unselected outpatients. In another study published in 2015, positive correlation between RDW and inflammatory markers is also observed in healthy population (5). These two studies clearly indicate that RDW is an inflammatory marker, and the association between RDW and cardiovascular diseases is at least partially mediated by inflammatory response.
It is well known that the development of autoimmune diseases is associated with chronic inflammatory response. Therefore, inflammatory markers are usually used to estimate the activity in many types of autoimmune diseases. Knowing that RDW is associated with inflammatory response, it is reasonable to speculate the RDW is a potential marker to estimate the disease activity of autoimmune disease. To date, many studies have reported that RDW is increased in autoimmune disease and correlates with disease activity. In this review, I summarized the studies concerning RDW and autoimmune disease.
RDW and rheumatoid arthritis (RA)
In 2010, Lee et al. reported that patients with RA had increased RDW when compared with osteoarthritis patients (6). RDW was further proved to be positively correlated with CRP in RA patients, regardless of anemia. In another studies (7,8), the researchers found that RDW is positively correlated with DAS-28, a widely used disease activity tool for RA. However, the correlation between RDW and DAS-28 was not observed in the study performed by Rodríguez-Carrio et al. (9). This may be due to the small sample size and lower statistical power. Taken together, these studies indicate that RDW is a potential index in estimating the disease activity of RA.
Increased RDW in RA patients was reported to be associated with risk of cardiovascular diseases. Rodríguez-Carrio et al. found that increased RDW, as well as RDW changes in the first year after diagnosis, is correlated high risk of cardiovascular events (heart failure, ischaemic heart disease or cerebrovascular accident), and the correlation remained significant after adjusting for sex and gender (7). In a cross-section study with 106 RA patients, Zhou et al. found that only creatinine, ESR, high sensitive CRP (hs-CRP) and RDW are associated with myocardial infarction (MI); however, in a multivariable logistic regression model, only RDW is independently associated with MI (10). Another study with a larger sample size (n=20,810) also found that increased RDW is associated with higher risk of MI, regardless of anemia (11). Taken together, these studies indicate that increased RDW in RA patients is associated with high risk of cardiovascular events.
The mechanisms underlying the relationship between RDW and risk of cardiovascular diseases remain largely unknown. A recent study found that RDW is negatively associated with endothelial progenitor cells (EPC) and serum interferon-α in RA patients, and the correlation between RDW and EPC remained even after adjusting for clinical parameters, disease duration, treatments and traditional cardiovascular risk factors (9). These results indicate that endothelial damage and impaired vascular repair mediates the relationship between increased RDW and cardiovascular events in RA patients.
RDW and systemic lupus erythematosus (SLE)
In 2013, Vayáet al. firstly reported that SLE patients have higher RDW than healthy controls (12). Although some of the SLE patients (26/105) has anemia, the SLE patients without anemia also showed higher RDW than healthy controls (12), indicating that increased RDW in SLE patients is not completely attributed to anemia. This study also indicated that RDW is positively correlated with CRP and ESR. Subsequent two studies also revealed that RDW is positively correlated with CRP and ESR (13,14). Besides, these two studies also found that RDW is positively correlated with SLEDA-2000K, C3, C4 and anti-dsDNA antibody (13,14). Increased RDW in SLE seems not to be associated with pharmacotherapy, because treatment naïve patients also showed increased RDW (13).
RDW also correlates with therapeutic outcome in patients with SLE. Compared to patients with normal RDW, patients with higher RDW have lower response rate to first line therapy, as well as lower one-year flare-free survival (14).
RDW and inflammatory bowel disease (IBD)
The first report concerning RDW and IBD was published in 2008. Clarke et al. reported that patients with Crohn’s disease (CD) have significantly higher RDW than patients with ulcerative colitis (UC) (15), but whether the CD or UC patients have higher RDW than healthy controls is not investigated. In 2009, Cakal et al. reported that patients with CD or UC have higher RDW than healthy controls (16). A subsequent study revealed that RDW represents a novel, inexpensive and promising index in estimating the disease activity of UC and CD, because active patients have higher RDW than patients in remission (17-21). Using receiver operating characteristic (ROC) curve analysis, these studies (16,18,19,22) revealed that RDW is a useful index in predicting the disease activity of UC and CD. Some of these studies even found that the area under ROC curve (AUC) of RDW is comparable to that of traditional inflammatory markers, such as ESR and CRP (16,18). Using multivariable logistic regression method, some studies also indicated that RDW is independently associated with active UC or CD, even after adjusting for CRP and ESR (20,22).
Increased RDW in IBD may be attributed to two possible mechanisms. One is inflammation response, which may impair erythropoiesis and accelerate newer, larger reticulocytes to enter the peripheral circulation (23). Another is iron deficiency, which is caused by malabsorption of iron. Indeed, iron deficiency is common in IBD patients and some studies found that RDW is associated with iron status markers (e.g., transferrin saturation, soluble transferrin receptor) in IBD patients (17).
RDW and ankylosing spondylitis (AS)
Three studies have investigated the association between RDW and AS (24-26). Two of these studies found that RDW is increased in AS patients (24,25). More importantly, RDW is positively correlated with the disease activity of AS, which is measured using Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). However, another study performed by Bozan et al. did not proved that AS patients had increased RDW (26). This may be due to the fact that the sample size in that study is small (n=30) and the statistical power is low.
RDW and primary Sjogren’s syndrome (pSS)
To present, only one study investigated the value of RDW in pSS (27). The authors found that pSS patients have higher RDW than healthy controls. RDW is positively correlated with ESR, CRP, as well as Sjogren’s Syndrome Disease Activity Index (SSDAI), a global index for estimating the disease activity of pSS. More importantly, RDW is independently correlated with SSDAI after adjusting for CRP, ESR and neutrophil to lymphocyte ratio (NLR).
RDW and inflammatory myopathy (IM)
Only one study has investigated the association between RDW and polymyositis (28). The authors found that RDW is increased in polymyositis and is associated with manual muscle test (MMT) score. In a multivariable logistic regression model, RDW is independently associated with polymyositis after adjustment for age, CRP, ESR, hemoglobin and leukocyte.
RDW and primary biliary cholangitis (PBC)
Because PBC is an autoimmune liver disease and RDW is reported to be increased in autoimmune disease as well as liver disease. The relationship between RDW and PBC is complicated. A study published in 2013 reported that RDW is increased in PBC and positively correlates with Mayo risk score (23). Subsequent studies found that RDW is increased as the advance of liver biopsy results in PBC patients (29). The AUC of RDW for predicting histologic severity of PBC was 0.66, which was comparable to that of aspartate aminotransferase to platelet ratio index (APRI) and Fib-4 (29). However, another study performed by Tahtaci et al. failed to observe the association between RDW and histological stage of PBC (30). Sources of inconsistency between these two studies remain largely unknown. In a recent study, RDW was reported to be affected by liver function tests of PBC (31). Since liver function tests and histological stage are well-known prognostic factors for PBC, it is reasonable to conclude that RDW is potential prognostic factor in PBC.
RDW and multiple sclerosis (MS)
To present, only one study has investigated the association between RDW and MS (32). Like other autoimmune diseases, RDW is increased in MS patients, and positively correlates with Expanded Disability Status Scale score and RDW decrease significantly in treatment responders.
Accumulated studies have proved that RDW is increased in autoimmune diseases and can be served as a useful index to estimate the disease activity of various autoimmune diseases, as summarized in Table 1. Although many biomarkers have been used to estimate the disease activity of autoimmune diseases, such as CRP and ESR, RDW has some advantages. First, RDW can be obtained from routine hematological tests, without any additional cost. Second, traditional markers, such as CRP and ESR, are sensitive to infectious events, while RDW is only affected by the size of red blood cell. Because the half-life of red blood cell is approximately 130 days, RDW appears to be insensitive to infectious events. In my opinion, RDW represents long-term inflammatory status, while CRP and ESR represent short-term inflammatory status.
It is noteworthy that almost all of available studies are cross-section study. Although these studies have proved that RDW is a useful index to estimate activity of autoimmune diseases, the prognostic value of RDW remains unknown. Further prospective cohort studies are needed to address the relationship between RDW and outcomes of autoimmune diseases.
Conflicts of Interest: The author has no conflicts of interest to declare.
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Cite this article as: Hu ZD. Red blood cell distribution width: a promising index for estimating activity of autoimmune disease. J Lab Precis Med 2016;1:4.