Clinical and hematological parameters of OSAS patients are shown in Table 1. A total of 73 male patients with OSAS were included, where 36 were mild (49.3%), 10 were moderate (13.7%), and 27 were severe (37%). There were not statistically significant differences between mild and moderate–severe groups regarding demographic parameters, medical history and Epworth sleepiness scale score.

Considering hematological parameters, only RDW was statistically higher in moderate–severe OSAS patients compared to mild (p=0.029). The mean RDW increased significantly with OSAS severity, the difference between mild and moderate–severe groups being statistically significant (p=0.029). In addition, RDW showed a positive mild correlation with RDI, ODI, and T90 and a negative mild correlation with lowest SpO2 (Table 2), and was more affected by ODI.

Clinical and hematological parameters of OSAS patients under treatment are shown in Table 3. A total of 48 OSAS patients underwent PAP therapy, where 18 were mild (37.5%), 5 were moderate (10.4%), and 25 were severe (52.1%). There were not statistically significant differences between mild and moderate–severe groups regarding demographic parameters, medical history, Epworth sleepiness scale score, and PAP compliance. However, BMI was higher in moderate–severe group compared with mild group (p=0.023).

Considering hematological parameters, there were not statistically significant differences between mild and moderate–severe groups

After six months of compliant PAP treatment (Table 4) the hemogram data, although showing normal reference values, revealed a significant decrease in the RBC count, hemoglobin, hematocrit, and platelet count in all patients (p<0.0001; p<0.0001; p=0.001; p<0.0001; respectively). Considering each severity group, these same parameters significantly decreased after PAP treatment in mild patients (p=0.003; p=0.043; p=0.020; p=0.014; respectively) and in severe patients only hemoglobin, hematocrit, and platelet count decrease significantly (p<0.0001; p=0.008; p=0.018; respectively).

In conclusion, concerning hematological evaluation in OSAS patients before PAP therapy, only RDW showed statistical increase according to OSAS severity. However, after six months of PAP therapy RDW values had not changed, which may be due to a small sample size.

There has not been consensus in previous studies in literature about RDW expression in OSAS. Some authors showed that RDW values were higher in OSAS patients than in controls13,14 and also higher in those with cardiovascular diseases.13 While others reported that RDW mean values were similar in OSAS patients compared to snorers.16

Considering these facts, we intended to evaluate the expression of RDW according to OSAS severity and in our study RDW increased significantly with severity of OSAS. Additionally, RDW showed a positive correlation with RDI and hypoxemic burdens (ODI, T90 and lowest SpO2). The exact mechanism of these results is not clear; however, this may be related to the existence of chronic inflammation. In fact, chronic inflammation promotes red blood cell membrane deformability and changes in erythropoiesis, thus increasing RDW.25

Moreover, the fact that RDW was also associated with hypoxemic burdens could be explained by the effect of hypoxia. In OSAS, sustained hypoxia, leads to activation of hypoxia inducible factor 1 resulting in increased erythropoietin expression26 and consequently higher RDW. These results, taken altogether, support our proposed role of RDW as a simple surrogate marker for OSAS severity.

A previous study, which compared controls with OSAS patients, reported a significant correlation between RDW and the apnea-hypopnea index (AHI), age, and mean SpO2.13 Also, the study of Sökücü and co-workers found that RDW was higher in patients with OSAS and increased significantly with severity, even after correction for anemia.14 Recently, Gunbatar and coworkers showed that RDW in OSAS patients was similar compared to snorers. They, also, reported that BMI, AHI, pulmonary artery pressure, and T90 were positively correlated with RDW in patients with OSAS.16 However, age and BMI were different between patients and controls, which could be confounding factors. Another recent study reinforced the idea that RDW could be a marker for OSAS severity27 because it was positively correlated with AHI, ODI, Epworth sleepiness scale, hematocrit and negatively correlated with minimum SpO2 and rapid eye movement sleep. However, in this multivariable analysis, only ODI was an independent predictor of RDW, which means a higher ODI will predict a higher RDW.

Additionally, in our study other hematological parameters did not significantly change with OSAS severity. In literature there are controversial results about the expression of these parameters in OSAS.16–19 Some authors have reported that the hematocrit, platelet count, MPV, and PDW increased in OSAS and correlated positively with severity,27 even after controlling for possible confounding factors. However, OSAS patients usually do not show clinical polycythemia.28 Our results, concerning the PDW and MPV tendency to increase with OSAS severity, could be explained by increased platelet activation and aggregation.29,30 In fact, MPV is an indicator of platelet activation which could result from sympathetic overactivity,31 hypoxia,32 and inflammation,33,34 all being well-known features of OSAS.2,3,35 Additionally, platelet activation may contribute to the increased incidence of cardiovascular events in patients with OSAS.36,19

After six month of PAP treatment, patients showed a significant decrease in RBC count, hemoglobin, hematocrit, and platelet count. Sustained hypoxia results in increased expression of erythropoietin26 inducing erythropoiesis with consequent increase in hematological parameters. PAP correction of respiratory events and consequent hypoxia and inflammation can translate in a decrease in RBC and platelets counts, hemoglobin, and hematocrit as obtained in our study. On the other hand, the tendency to decrease MPV and PDW could be also explained by the fact that besides PAP decreasing hypoxia and inflammation, it also improves platelet aggregability.37

Our results agree with previous studies that showed decreases in hematocrit after PAP treatment.38,39 A recent study20 reported a significant reduction of hematocrit and MPV, while RDW and PDW increased after six months of PAP treatment in patients with severe OSAS.

There are potential limitations of this study, such as the small sample size, exclusion of women, and lack of comparison with a group of OSAS patients under sham PAP. Also, in this study we did not evaluate the correlation of some markers of inflammation, neurohormonal activation, or oxidative stress with these hematological parameters. The lack of exclusion of OSAS patients with hypertension, diabetes or chronic medication such as anti-coagulation, anti-aggregation, anti-inflammation or immunosuppression agents is also an additional limitation. Finally, although our study population was not anemic, we did not measure nutritional status, which could be a potential cause of increased RDW.

The authors declare that no experiments were performed on humans or animals for this study.

The authors declare that they have followed the protocols of their work center on the publication of patient data.

The authors have obtained the written informed consent of the patients or subjects mentioned in the article. The corresponding author is in possession of this document.