Objective:To analyze the relationship between blood electrolytes and the prognosis of patients with severe coronavirus disease 2019 (COVID-19) and to provide assistance for clinical decision-making.Methods:The clinical data of patients with severe COVID-19 admitted to intensive care unit (ICU) of the Wuhan Third Hospital by the Shanghai aid-Hubei medical team from January 21 to March 4, 2020 were collected. Excluding ineligible patients, 110 patients were finally enrolled. The patients' gender, age, temperature, heart rate, systolic and diastolic blood pressure, clinical symptoms at admission, time of symptom onset, duration of fever, and relevant indicators at admission to ICU (including blood potassium, chloride, sodium, calcium, phosphorus, and magnesium, etc.) and prognosis were analyzed. The patients were grouped by blood potassium or calcium levels or blood potassium/calcium ratio. The Kaplan-Meier survival curves were used to analyze the survival of patients in each group. The relationship between the potassium/calcium ratio and the prognosis was analyzed using restricted cubic spline plots. The relationship between each index in the different models and the prognosis was analyzed using Cox regression models.Results:Among 110 severe COVID-19 patients, 78 cases survived, and 32 cases died. Compared with the surviving group, patients in the death group had higher blood potassium levels [mmol/L: 4.25 (3.80, 4.65) vs. 3.90 (3.60, 4.20), P < 0.05] and lower blood calcium levels (mmol/L: 2.00±0.14 vs. 2.19±0.18, P < 0.05). The Kaplan-Meier survival curves showed that patients in the potassium > 4.2 mmol/L group had a worse prognosis than the potassium < 3.8 mmol/L group and the potassium 3.8-4.2 mmol/L group ( P = 0.011), patients in the calcium > 2.23 mmol/L group had a better prognosis than the calcium < 2.03 mmol/L group and the calcium 2.03-2.23 mmol/L group, and the lower calcium group had a worse prognosis ( P = 0.000 15). Cox regression analysis showed that the hazard ratio ( HR) of blood potassium and calcium were 2.08 and 0.01, respectively, in model 1 (single blood potassium or calcium) and in model 2 (model 1 plus age and gender), the HR of blood potassium and calcium were 1.98 and 0.01 respectively, which were significantly associated with patient prognosis (all P < 0.05). Patients in the group with the potassium/calcium ratio > 1.9 had higher blood potassium levels and a higher proportion of mechanical ventilation, lower calcium levels and lower proportion of survival, and longer time of ICU admission compared with the groups with the potassium/calcium ratio < 1.7 and 1.7-1.9. The Kaplan-Meier survival curves showed that the survival rate of the potassium/calcium ratio > 1.9 group was the lowest ( P < 0.000 1), and there was no statistically significant difference in survival between the potassium/calcium ratio < 1.7 group and the potassium/calcium ratio 1.7-1.9 group. A restricted cubic spline plot corrected for age and gender showed that patients in the potassium/calcium ratio > 1.8 group had HR values > 1. Cox regression analysis corrected for other indicators showed that the potassium/calcium ratio was still associated with patient prognosis ( HR = 4.85, P = 0.033). Conclusions:Blood potassium, calcium, and the potassium/calcium ratio at ICU admission are related to the prognosis of patients with severe COVID-19, and the potassium/calcium ratio is an independent risk factor for the death of patients. The higher the potassium/calcium ratio, the worse the prognosis of patients.