The main finding of the study is that baseline demographic and laboratory findings can predict the outcome of treatment in hospitalized patients with SARS-CoV-2 infection. The created model of mortality risk assessment is based on objective tests carried out on admission to the hospital. Early evaluation of in-hospital mortality is important, because if followed, it may improve health awareness of and facilitate the identification of high-risk individuals. Therefore, since it is related to the optimization of management strategies, learning the unfavorable factors is more important than simple forecasting.
In the COVID-19 era, many models for predicting in-hospital mortality have been built [9–22]. The data used to create them usually include age, gender, and selected elements of the treatments used, comorbidities, and results of radiological and laboratory tests. The collection of a detailed medical history sometimes is not easy to obtain, given that we focus on shortening patient contact time. The comorbidities are present in many mortality prediction models. Typically, the severity of a comorbidity is more significant than its presence itself. Therefore, we did not include comorbidities in the created model, as their impact is reflected in laboratory tests. We also did not include radiological screening, because, in clinical practice, a chest computed tomography is not a routine on-admission procedure used in patients with SARS-CoV-19 infection, especially given the circumstances of the pandemic. Oxygen saturation in room air is another important prognostic factor, but in real life, in patients undergoing oxygen therapy, it is difficult to obtain. The shortcoming of same prediction scores is inaccessibility of laboratory tests like ferritin or interleukine-6. Furthermore, risk scores developed in one population may not work in another, due to ethnicity variables, availability of treatment resources, and other, yet unidentified factors.
The presented risk score was developed as easy-to-implement and observe-independent, hence we decided to include only demographic data and basic laboratory tests.
As in other published reports, in this study, male gender is a factor that predisposes to the risk of death. Older age is also associated with a higher risk of death, as found by all authors of the COVID-19-related mortality study [10, 11, 13–16]. Age differences between cohorts in our study may be related to the different time of cohort formation. The tendency to admit patients to the hospital at the beginning of the epidemic was higher because the degree of risk related to COVID-19 infection was unknown [23].
The laboratory abnormalities in patients with COVID-19 disease are common. They could be related either to the disease itself, or to the comorbidities often leading to a severe course of COVID-19, such as: all types of malignancies, cardiovascular diseases, chronic kidney diseases, chronic obstructive pulmonary disease, obesity (BMI > 40), pregnancy, type 2 diabetes mellitus, sickle cell anemia and medications used.
Anemia, leukocytosis, increased CRP, creatinine, low or high sodium, potassium levels or platelet count is related to disturbed homeostasis and represents changes that could be related to SARS-CoV-2 infection or concomitant diseases and their treatments. The mortality in COVID-19 patients is higher when the comorbidities are present [5, 6, 15]. It should be stressed that the comorbidities often present as laboratory abnormalities even in patients without SARS-CoV-2 infection.
The significance of selected demographic and laboratory factors for the outcome is confirmed in many studies.
Anemia in patients with SARS-CoV-2 could be present before infection due to chronic illness, especially malignancies. Anemia could develop in the course of a SARS-CoV-2 infection by inflammation alone, including direct cytopathic injury of circulating erythrocytes or/and their bone marrow precursors due to infection, the damage due to hemolytic anemia, and/or thrombotic microangiopathy. The presence of anemia is related to increased mortality [24] which is concordant with the presented study.
In our study, surprisingly, higher MCV level is also related to the increased risk of mortality. The reason could be diseases which show increased MCV, such as hypothyroidism or vitamin B12 deficiency. However, further studies are required to assess whether elevated MCV level found during infection is the same as before the disease or not. The latter could indicate might result in MCV changes [25]. Inconsistently to our observation, the MCV in COVID-19 patients was lower than in healthy participants [26]. However, the MCV in non-survivors increased during hospitalization, resulting in mean MCV values similar to those presented in our study.
The leukocyte count is an important parameter used to predict the severity of COVID-19 disease [27]. Huang et al. found that ICU patients with COVID-19 disease had more leukocytes than non-ICU patients [28]. SARS-CoV-2 infection is primarily related to lymphopenia what may decrease leukocyte numbers, however, later on neutrophil count increases, leading to leukocytosis.
Increased CRP level and leukocytosis are signs of infection severity and according to the expectations are related to increased mortality [29, 30]. Increased CRP level is present in many risk scores predicting mortality in patients infected SARS-Cov2 but its cut-off point is different in various study populations [10, 13, 16–18, 21]. The CRP cut-off point established in this study was at 51.5 mg/dL.
The significance of platelet count for the outcome prediction was also studied in many groups of patients. In some other studies, platelets did not vary between survivors and non-survivors. However, the reason behind this could have been the presence of patients with both low and high platelets in the studied group; this, in turn, could lead to their normal average count [31]. Furthermore, Lippi et. al. found that thrombocytopenia is related to a higher risk of adverse events during hospitalization [32].
Hyponatremia is a common finding in pneumonia patients regardless of the etiology of the disease [33]. However, it is more prevalent in SARS-CoV-2 infected patients than in patients with pneumonia of other origins [34]. Our observation that hyponatremia is related to adverse outcomes in COVID-19 patients is concordant with other studies [35]. Hyponatremia is an outcome of various mechanisms, including an induction of the non-osmotic release of vasopressin by IL-6, which is increased in COVID-19 patients and inversely related to hyponatremia [36].
Hypernatremia, although rare, is also encountered in patients with SARS-CoV-2 infection and is related to higher mortality [37]. It may be the result of loss of free water due to perspiration. It may also be caused by abnormally increased renal sodium reabsorption due to increased angiotensin II activity, resulting from angiotensin-converting enzyme 2 receptors blockage by SARS-CoV-2 [38].
Hypokalemia is also commonly found in patients with COVID-19 pneumonia. Moreno-Perez reported that hypokalemia is a sensitive biomarker of adverse COVID-19 progression [39]. In the presented study, hypokalemia was a factor indicating poor outcome. Additionally, high potassium level was an unfavorable factor, which may be associated with renal failure and treatment with potassium-sparing drugs, which may be an indicator of comorbidities.
Creatinine level was the next predictor for adverse outcome. This is concordant with the findings of other authors [5, 10, 37]. Yang et al. reported that nearly 30% of COVID-19 patients with severe pneumonia showed increased creatinine [40]. High creatinine level in patients with COVID-19 may be a sign of their concomitant diseases, or may suggest that SARS-CoV-2 is able to induce kidney disease.
The validation cohort of the Covid19-score showed similar accuracy and prognostic results as the developing cohort. However, it could be noted that the two studied populations differed in terms of age and gender distribution. The difference may result from the hospitals' location: the developing cohort was hospitalized in a small town, while the validation cohort was hospitalized in a larger city, province capital. Furthermore, dialyzed patients with SARS-CoV-2 infection were admitted only to the hospital in Boleslawiec, where they were put on dialysis irrespectively of the presence of infection symptoms. Close accuracy of the Covid19-score in those two different populations indicates the significance of laboratory abnormalities.
The prediction of mortality of COVID-19 patients was assessed by other authors which used Chinese protocol severity classification, the pneumonia severity index (PSI), and Confusion-Urea-Respiratory Rate-Blood pressure-65 (CURB-65) in risk stratification and prognostic assessment. The AUC of the Chinese protocol severity classification, PSI, and CURB-65 was 0.735, 0.951, and 0.912. The AUC of the presented scores is similar to our study [23].
Studies based on results coming from a single laboratory facility had lower predictive accuracy. For example, lung ultrasound results did not predict mortality [12]. The degree of lung involvement may be considered important, but not critical to survival, which depends rather on body’s overall response to the infection.
In the presented study we do not exclude parameters that seem to be only borderline-related to mortality in the developing cohort. The significance of the assessed parameters may vary between different populations, therefore, the wider range of research makes it easier to predict the outcome. It is worth noting that the optimal cut-off slightly differs between the developing and validation cohorts. Furthermore, it is the same in developing and prospective cohorts. This finding may suggest that the importance of analyzed parameters differs in different populations – it needs further investigation.
The mortality in COVID-19 patients could be also related to the capabilities of the health care system, for instance early diagnosis, as the number of performed tests could play a significant role.
The clinical benefits of using the Covid19-score may be helpful in the pandemic period, when the number of patients requiring hospitalization is very high and it is necessary to list patients with a high risk of death upon admission. The particular value of the created scoring model is the fact that the model objective and based on simple diagnostic tools and methods widely available in all hospitals. The model might be especially useful in field and mobile hospital facilities. Such an initial assessment with the use of Covid19-score facilitates making decisions regarding the diagnostic and therapeutic management strategies in patients with SARS-CoV-2 infection. In addition, application of the Covid19-score could help compare the effectiveness of treatment using different methods.
Limitations
The main limitation of the study is the lack of clinical data related to comorbidities and treatment used. However, the aim of the study was to construct a simple threat-assessing score that could be used for individual patients even by professionals less familiar with clinical assessment of COVID-19 patients.
The other limitation is the lack of data regarding the time from the onset of the first symptoms to hospital admission. Laboratory findings could change in the course of the infection and the usage of the scores obtained at the different points of the infection process may be not adequate.
The third problem was that the inclusion criterion for the study was a positive COVID-19 result during hospitalization. Some patients may be infected while hospitalized, and abnormal laboratory test results on admission may not necessarily point to a SARS-CoV-2 infection, but can be related to other diseases. SARS-CoV-2 infection can develop during hospitalization, also as asymptomatic. Additionally, some SARS-CoV-2 infections may not be revealed during the first smear test. Including only patients who had a positive test result on admission could omit such patients.