Dear Editors,

I came across a very interesting article published in your esteemed journal titled ‘Investigation of the relationship between neutrophil-to-lymphocyte ratio and obstructive sleep apnoea syndrome’ by Yenigun et al.Reference Yenigun and Karamanli¹ The article is well written and I appreciate the effort put in by the authors. However, I am concerned about the authors’ conclusion and would like to express the same through your prestigious journal.

The increase in inflammatory markers and its effect on cardiovascular morbidity in obstructive sleep apnoeaReference Jiang, Cao, Wang, Liu, Cao and Chen² has been known for quite some time. Neutrophil-to-lymphocyte ratio presents an exciting option for monitoring obstructive sleep apnoea patients because it is very simple to determine, and is not uncomfortable and cumbersome like polysomnography. However, a word of caution is advised, for multiple reasons.

Firstly, neutrophil-to-lymphocyte ratio varies with patient age and body mass index,Reference Li, Chen, Luo, Hong, Pan and Lin³ and will therefore vary from person to person. There are no standard cut-off values for neutrophil-to-lymphocyte ratio to determine whether it is abnormal or not. Without such standardisation, it is difficult to determine the significance of neutrophil-to-lymphocyte ratio in these patients.

Secondly, although abnormal neutrophil-to-lymphocyte ratio in obstructive sleep apnoea has been proven in previous studies,Reference Korkmaz, Korkmaz, Küçüker, Ayyıldız and Çankaya⁴ there are many other common conditions, such as Bell's palsyReference Kum, Yurtsever Kum, Ozcan, Yilmaz, Gungor and Unal⁵ and even chronic tonsillitis,Reference Yenigun⁶ where the values of neutrophil-to-lymphocyte ratio can be deranged. As the neutrophil-to-lymphocyte ratio can be influenced by so many common conditions, its specificity for use as a diagnostic tool comes into doubt.

Thirdly, although the authors have concluded that neutrophil-to-lymphocyte ratio correlates with the severity of obstructive sleep apnoea, it is not clear whether this relationship is linear or not. Only if the relationship is linear can it be used as a diagnostic tool for determining the severity of obstructive sleep apnoea. Another point which could have been evaluated is whether the neutrophil-to-lymphocyte ratio also depends on the duration of obstructive sleep apnoea and other co-morbid factors such as hypothyroidism. This can be done in future prospective trials.

I would like to conclude by stating that we still have a long way to go before we can truly analyse the significance of neutrophil-to-lymphocyte ratio in obstructive sleep apnoea.