Master thesis title: Nursing care in support of C-19 treatment of elderly patients – influence of vitamin D concentraton on disease outcome Objectives: To investigate is it possible to predict the outcome of the disease of COVID-19 in the elderly population with the help of analyzed laboratory parameters. Data sources and methods: The research involved 176 respondents, i.e. people over 60 years old who were hospitalized in the intensive care unit of KBC Split due to COVID-19 infection, of which 63 died and 113 survived. Venipuncture of serum samples was performed in accordance with the National Recommendations for Venous Blood Sampling. Vitamin D levels were analyzed by Roche immunochemical electrochemoluminescent method. The analysis of all collected data was performed using the MedCalc computer program, and the level of statistical significance was set at p < 0.05. Results: The research showed that there is a statistically significant difference between the deceased and the deceased with regard to age (p=0.016), vitamin D level (p<0.001), calcium level (p<0.001) and magnesium level (p=0.001). The level of vitamin D is statistically significantly higher in survivors compared to deceased patients (22.8 ± 20.8 vs. 36.6 ± 22.8; p< 0.001). The level of GDF-15 is also higher in survivors, but this difference is not statistically significant. It was shown that there is a statistically significant relationship between the levels of magnesium and calcium and the outcome of the disease of COVID-19 in the elderly population, but not a statistically significant relationship between the levels of phosphorus and the number of leukocytes, neutrophil granulocytes, lymphocytes and platelets and the outcome of the disease. Statistical processing indicated a connection between GDF-15 and magnesium (in the total sample, in the group of survivors and deceased), phosphorus (in the total sample and in the group of survivors) and calcium (in the group of deceased). It was also shown that there is a correlation between vitamin D and calcium (in the total sample, in the group of survivors and deceased) and magnesium (in the total sample and in the group of the deceased), while no connection was observed with phosphorus. Multivariable logistic regression analysis showed that only vitamin D level in elderly patients with COVID-19 infection is lowered (OR 1.047, 95% CI 1.020 - 1.075; p<0.001) when put into the model with age and GDF-15 and catestatin levels and the number of leukocytes. Conclusion: The outcome of the disease of COVID-19 is worse the older the patient and the lower the concentration of vitamin D in the patient's blood. Magnesium and calcium levels can also affect the outcome of the disease, but their correlation with the outcome of the disease of COVID-19 should be examined in more detail.