DISCUSSION
In recent years, researchers have examined the value of ratios such as NLR, PLR, and monocyte-lymphocyte ratio (MLR) in the diagnosis and prognosis of several inflammatory conditions.
The results of this study demonstrated that there was a relationship between CRP, NLR, and PLR, and the severity score of lung involvement according to thoracic CT findings in patients with COVID-19 pneumonia.
NLR in the peripheral blood is associated with disease activity and the systemic inflammatory status, and has been shown to have prognostic value in autoimmune diseases, malignant tumours, and infectious diseases (12-15).
In a previous study of 61 COVID-19 patients, NLR was reported to be the most useful marker showing the incidence of severe COVID-19, with a cutoff value of ≥3.13 for the differentiation of severe and non-severe disease(16).
Shang et al evaluated the clinical data of 443 COVID-19 patients and found that NLR, CRP, and thrombocytes could be useful in determining the severity of disease, and although NLR was the best determinant, all the parameters should be taken into consideration (17). PLR, which is a new inflammation index, reflects the level of systemic inflammation.
Previous studies have reported that PLR is closely associated with tumours, diabetes, coronary heart disease, and connective tissue diseases, and an increase in PLR is related to tumour size, lymph node infiltration, distant metastasis, and prognosis, and could be used as a potential inflammatory marker for the clinical diagnosis of community-acquired pneumonia (18).
In a study of 30 patients diagnosed with COVID-19, Qu et al suggested that an elevated PLR in the blood parameters prolonged the length of stay in hospital and there could be a relationship with disease prognosis (19).
The results of the current study demonstrated a weak positive correlation between the lung involvement severity score determined on CT and the PLR and NLR in patients who developed COVID-19 pneumonia.
CRP is an acute phase protein synthesised by hepatocytes. In lung diseases with inflammatory characteristics, the serum CRP level is generally increased as a response to inflammatory cytokines such as interleukin-6 (IL-6), IL-1, or tumour necrosis factor alpha (TNF-α) (20, 21).
In COVID-19 patients, CRP increases as a response to pro-inflammatory cytokines and the majority of previous studies have reported this increase (22, 23).
In a previous study, CRP levels in COVID-19 patients on hospital admission were a determining factor independently of disease severity during hospitalisation, and CRP was reported to show good performance in the prediction of negative outcomes in COVID-19 patients (24).
In the current study, a moderate level correlation was observed between CRP levels on admission to hospital and the lung involvement severity score.
In daily practice, NLR, PLR, and CRP are lower cost markers than cytokines because blood count is widely used in the clinic routine. Therefore, NLR, PLR, and CRP on first admission to hospital can be used as a useful marker of the extent of radiological involvement. The use of these ratios in the prediction of the severity of the radiological involvement can provide both early intervention and reduce the exposure to ionising radiation required by thoracic CT.
Limitations of this study could be said to be the relatively small patient population, that it was conducted in a single centre, and the retrospective design. However, to the best of our knowledge, it is the first study to have been conducted on this subject.
The results of this study demonstrated that it can be concluded that in COVID-19 patients with higher NLR, PLR, and CRP values, the lungs will be more affected radiologically. Nevertheless, there is a need for further studies on this subject.