Nephrol Dial Transplant (2003) 18: 1948
© 2003 European Renal Association-European Dialysis and Transplant Association
Letter and Reply
Reply
Queen’s University Kingston Ontario Canada Email: eai1{at}post.queensu.ca
Sir,
Thank you for the opportunity to comment on the letter of Sabbatini et al. in the current issue of the Journal, regarding the association between haemoglobin level and quality of sleep in haemodialysis patients. The authors measured haemoglobin level and quality of sleep using the Pittsburgh Sleep Quality Index (PSQI) in a cross-sectional study of 249 prevalent haemodialysis patients. The authors compare the results to those of our study of 89 prevalent haemodialysis patients [1], and seek to explain the differences. In both studies the mean haemoglobin level was higher in subjects with PSQI 
5 (good sleep) compared with those with PSQI >5 (poor sleep). In our study the haemoglobin level was weakly but significantly correlated with PSQI (Spearman r = -0.27, P < 0.01). In the Sabbatini study this correlation was weak, and did not reach statistical significance (r = –0.09, P = 0.17). Sabbatini et al. found that haemoglobin was not an independent predictor of PSQI (5 vs >5) in multiple logistic regression adjusting for dialysis unit and age.
In bivariate analysis, the comparison of mean haemoglobin among categories of PSQI (5 vs> 5) is superior to the comparison of continuous variables by correlation because the PSQI is primarily intended as a categorical instrument to identify ‘poor sleepers’ [2], and because PSQI may not be normally distributed. In the categorical analysis the two studies have very similar results. In regards to the multivariate analysis, we agree with the authors that both studies were underpowered to examine the independent influence of haemoglobin on quality of sleep while controlling for the large number of possible confounding variables.
We conclude that the results of the two studies are similar. They support the hypothesis that haemoglobin influences quality of sleep. While there is evidence for biological plausibility [3], this association clearly does not satisfy the criteria for causation at this time. Inclusion of the PSQI and polysomnography as outcome variables in prospective studies that randomize patients to different levels of haemoglobin would bring us closer to the ‘truth’.
The similarity in the prevalence of poor sleep observed in the two studies, despite two culturally different populations, is remarkable and speaks to the validity of the PSQI as a tool for comparing sleep quality among different populations.
Conflict of interest statement. None declared.
References
- Iliescu EA, Coo H, McMurray MH et al. ‘Quality of sleep and health related quality of life in hemodialysis patients’. Nephrol Dial Transplant 2003; 18: 126–132
[Abstract/Free Full Text] - Buysse DJ, Reynolds CF 3rd, Monk TH, Berman SR, Kupfer DJ. The Pittsburgh Sleep Quality Index: a new instrument for psychiatric practice and research. Psychiatry Res 1989; 28: 193–213[CrossRef][Web of Science][Medline]
- Benz RL, Pressman MR, Hovick ET, Peterson DD. A preliminary study of the effects of correction of anemia with recombinant human erythropoietin therapy on sleep, sleep disorders, and daytime sleepiness in hemodialysis patients (The SLEEPO study). Am J Kidney Dis 1999; 34: 1089–1095[Medline]
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