Primary Health Care: Open Access

Table 2: Differences in hematological parameters and age according to group allocation (IDA and BTT).

The sensitivity, specificity, PPV, NPV, and YI of RDW and RDWI as discriminator indices are shown in Table 3 according to selected cutoff values. Overall, the RDWI demonstrated better sensitivity for the discrimination of BTT (83.18%), in contrast to RDW which had greatest sensitivity for the detection of IDA (93.66%). The highest specificity rate was obtained using RDW for the discrimination of BTT (69.58%). However, both discriminator indices had low YI values of <0.5. Nevertheless, when the cut-off value was decreased to 199, both the sensitivity and NPV of RDWI increased to 85.7% and 96.92%, respectively, for the detection of IDA. Similarly, the specificity of RDW as a discriminator index for the detection of IDA increased to 80.69% when the cut-off value was raised to 16.1%. High NPV rates persisted for both groups when the cut-off value for RDW was increased (92.07% and 81.15%, respectively).

Table 3: Sensitivity (TPR), specificity (TNR), PPV, NPV, and YI for RDW and RDWI as discriminator indices in the differentiation between IDA and BTT.

A ROC curve analysis was conducted using a cut-off value of 199 for RDWI and 16.1% for RDW. Figure 2, AUC values for RDWI and RDW were 0.81 and 0.62, respectively, thus indicating that RDWI was a better and more appropriate diagnostic discriminator index than RDW to differentiate between IDA and BTT in our population.

Figure 2. ROC curve plots showing the AUC for RDWI and RDW as discriminator indices in the differentiation between IDA and BTT.
AUC = Area Under the Curve, BTT = β-Thalassemia Trait, IDA = Iron Deficiency Anemia, RDW = Red Cell Distribution Width, RDWI = Red Cell Distribution Width Index, ROC = Receiver-Operating Characteristic, sens = Sensitivity, speci = Specificity.

Discussion

According to the WHO criteria, anemia is defined by a blood Hb value of <7.7 mmol/L (13 g/dL) in men and 7.4 mmol/L (12 g/dL) in women; however, serum ferritin measurement remains the preferred method for detecting depleted iron stores in cases of IDA [5,6]. Nevertheless, previous studies have utilized varying cut-off values of serum ferritin to diagnose IDA. For instance, both Jameel et al. and Nesa et al. used ferritin values of <12 ng/mL, whereas Soliman et al. and Jahangiri et al. used values of <15 and <28 ng/mL, respectively [24-27]. This wide variation in serum ferritin criteria for the diagnosis of IDA could affect sensitivity rates of discriminator indices when differentiating between IDA and BTT. In contrast, the majority of these previous studies used an HbA2 concentration of >3.5% to diagnose BTT, as was the case in our study as well [25-27]. However, Jameel et al. used an HbA2 concentration of >3.2% to detect BTT cases among 620 adults undergoing premarital screening [24].

In the current study, the IDA group demonstrated low median Hb and RBC values, but higher median MCV, RDW, and RDWI values. Multiple studies have reported similar findings [26,28,29]. Rosatelli et al. concluded that HbA2 levels were not related to the severity of the mutation in BTT cases of Italian descent and that this factor varied among individuals with different mutations [30]. On the other hand, Rund et al. found that MCV was directly related to BTT mutation severity among a group of adults and older children of multiple ethnicities in Israel [31]. In addition, IDA can cause a decrease in HbA2 level, which could confuse the diagnosis and cause BTT to be overlooked if both conditions coexist in a single patient [32]. These factors could affect the use of standardized cut-off values for discriminator indices and explain reported variations when these indices are used in different populations. As such, further research is needed in order to determine appropriate cut-off values for discriminator indices in the Omani population.

The theoretical foundation for the use of RDW as a discriminator index is as a measurement of the degree of anisocytosis in the red blood cells, with this being usually high in IDA and near normal or only mildly increased in BTT. Some studies have found RDW to be a useful discriminator index in differentiating between IDA and BTT [33-35]. However, in our study, RDW resulted in low YI and AUC values (<0.3 and 0.62, respectively), thus indicating this index to be a poor discriminator, despite its high sensitivity (93.66%) and NPV (96.2%) in the detection of IDA, as well as the significant difference in median values between IDA and BTT (16.6% versus 15.8%; P <0.001). Most previous studies have used cut-off points of 14% to discriminate between IDA and BTT using RDW [24,25,27]. Calculating the AUC for RDW in our study showed the maximum value occurred with a cut-off point of 16.1%, a value considerably higher than that used in our reference standard [21]. Nevertheless, despite using this higher cut-off point, the AUC value remained low at 0.62, signifying that RDW was a poor discriminator index in our population [23].

In line with these findings, Mohammed et al. reported that RDW had limited value for discriminating between IDA and BTT among a cohort of Egyptian children (AUC: 0.553), with a non-significant difference in mean RDW values between the two groups, despite using an even higher cut-off value (16.4%) [36]. Comparable results have also been reported among other populations in Saudi Arabia and India [24,33]. Furthermore, a study conducted in Iran evaluating 10 different discriminator indices found that red cell indices were affected by BTT mutation type, a factor which would hinder use of the same cut-off values for different populations prone to unique or different mutations [37]. Soliman et al. found that RDW at a cut-off point of 14.5% was able to differentiate BTT from IDA among 200 samples with microcytosis with 83.3% sensitivity and 100% specificity; however, MCV at 73 fL or less demonstrated better discrimination at 91.7% sensitivity and 100% specificity [26].

In our study, a higher AUC value of 0.81 was obtained for RDWI at a cut-off point of 199, thus indicating RDWI to be an excellent discriminator index for the differentiation of IDA and BTT at this threshold [23]. This finding was supported by the significant difference in median RDWI values between the IDA and BTT groups (228. 24 versus 188.15; P <0.001). Despite using a lower differential cut-off point compared to our reference standard, the sensitivity of RDWI in detecting IDA increased from 56.49% to 85.7% [22]. Similarly, the NPV increased from 89.46% to 96.92%. In contrast, the sensitivity of this index for detecting BTT dropped from 83.18% to 65.5%, despite maintaining a NPV rate of >90%. In turn, the YI value increased only minimally from 0.22 to 0.29.

In comparison, Demir et al. reported a YI of 0.8 using a cut-off value of 220 for RDWI, concluding that this index was one of the most reliable discriminator indices for the differentiation of BTT and IDA [38]. Despite reporting a lower YI (0.65), Nesa et al. also judged RDWI to be both reliable and useful and a better index than RDW in differentiating between the two conditions [25]. In a study of a mixed Sistani and Baloch population from Iran, Miri-Moghaddam et al. found a cut-off value of 212 to result in the highest accuracy (i.e. resulting in the greatest sum of sensitivity combined with specificity), with an AUC value of 0.819 [37]. The researchers also suggested a need to determine appropriate cut-off values for each discrimination index in different populations due to the effect of BTT mutations on red cell indices [37]. Likewise, Fonseca Motas et al. concluded that RDWI appeared to be a reliable and useful index for the initial screening of microcytic hypochromic anemia in a Brazilian population and was better than RDW in differentiating IDA from BTT, with an AUC value of 0.912 [39].

There are several limitations to the current study. First, cases in which both IDA and BTT were suspected to co-exist in a single patient were not excluded from the analysis. Second, the sample size was relatively small. Further research is needed, particularly studies involving a larger sample size and the evaluation of additional discriminator indices in the Omani population. Moreover, a larger sample would help to more accurately determine appropriate cut-off points for such indices in this specific population.

Conclusion

Based on the findings of this study, RDWI was an excellent discriminator index to differentiate between IDA and BTT in an Omani adult population, resulting in an AUC value of 0.81 at a differential cut-off of 199. In turn, RDW was a poor discriminator, even after adjusting the cut-off value from 14% to 16.1%. Additional research is recommended in order to more accurately confirm appropriate cut-off values for red cell indices in the general Omani population.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Acknowledgements

The authors would like to express their sincere gratitude to Dr. Sanjay Jaju, Assistant Professor of Epidemiology in the Department of Family Medicine and Public Health at the College of Medicine and Health Sciences at Sultan Qaboos University (SQU), and Dr. Fahad Al-Zadjali, Assistant Dean of the Deanship of Research at SQU, for their help in analyzing the data for this study.

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