This study is the first clinical study to describe the correlation and evolution of various laboratory parameters with the clinical severity and progression of COVID-19 based on clinical stages but not an absolute course of disease. Through multi-dimensional comparison of various parameters, we have found LDH as an indicator of both predictive and follow-up value in COVID-19 cases.
Although there have been many studies describing the clinical characteristics of COVID-19 patients and multiple possible prognostic indicators, we should pay attentions to the research methodology and process when evaluating those indicators. First, admission should not be used as a starting point for follow-up when evaluating prognostic indicators. Laboratory parameters are supposed to reflect the patient's clinical condition, but the patient's admission time is determined by many non-medical accidental factors. Therefore, analysis of the parameters at admission of all patients[3,5−7] undermines their correlations of the disease severity. Secondly, it is also inappropriate to follow up the laboratory parameters with an absolute course of disease. For example, in Zhou's article[3], the curve of D-Dimer and lymphocytes stayed relatively flat in first 7 days since the onset-of symptoms and began to rise significantly after that. This is consistent with Zhou’s finding that non-survivors of COVID-19 progressed to sepsis on the average of seven days. In another words, the non-survivors experienced several stages of the disease, i.e. mild, severe and even critical ill, in the first seven days and stayed in critical stages in the following 14 days. Therefore, the significantly elevated part of the curve described the patients in critical stage and these indicators were related to death, but not early predictors for disease progression. Third, predictors should show abnormalities earlier than the deterioration of the clinical condition manifested by the patient's symptoms and signs. The right way to find those indicators is to compare the parameters of severe cases in their mild stage to the persistent mild patients[4]. It is common to compare admission findings of the severe cases to the mild ones. However, as previously mentioned, many patients were seriously ill when they were admitted to the hospital. This kind of comparison would show the difference between severe and mild patients but not risk factors for developing severe cases. Fourth, for indicators that appear abnormal in most persistent mild cases, even if they also showed statistical differences to severe ones, it may have limited clinical application value for clinicians. These indicators need to establish a cut-off specific for COVID-19, and it is also affected by technical factors such as laboratory examinations, resulting a higher threshold for acceptance and application in clinical practice.
We summarized a “CDEF” rule for indicators, namely correlation, differentiation, early-warning, and feasibility. We measured the correlation by Spearman analysis and found that LDH, PCT, NT-proBNP, MYO and D-dimer correlated well to the severity of COVID-19. The ability of differentiating the clinical conditions was quantified as the AUC value of the ROC curve, and we noted the superior performance of LDH in differentiating between mild and severe patients. Early-warning was to show abnormalities even in the mild stage of the disease, therefor helping clinicians to find high-risk patients who might deteriorate. One way to achieve feasibility was to warn the clinicians when the indicators become abnormal, i.e. beyond the normal range, rather than another unestablished cutoff. In this study, CRP and D-dimer levels were above the upper limit of normal both in mild and severe cases although there were significant differences between mild and severe cases. Thus, above-normal CRP or D-dimer had the difficulty to indicate the disease progression.
Lactate dehydrogenase is a cytoplasmic glycolytic enzyme found in almost every tissue. Its elevation generally indicates tissue damage. Raised LDH is a common finding in patients infected with MERS-CoV[8, 9], H7N9[10, 11] and H5N1[12]. It is reported to be independent factors of mortality for patients with severe acute respiratory syndrome[13] and H1N1 infection[14]. It is also one of the biomarkers most strongly associated with ARDS mortality[15,16]. Our research did not combine LDH with other indicators. The first reason is that LDH's ROC AUC for predicting severity of COVID-19 is more than 0.95. The combination of other indicators that are inferior to LDH is of limited significance for improving prediction performance. The main significance of the early predictors is to identify high-risk patients in order to allocate medical resources more rationally and improve the prognosis, but not predicting the prognosis itself. Therefore, we believe that even if LDH alone may slightly inferior to indicator combination in the predicting accuracy, it can greatly improve the convenience in clinical practice. What’s more, the present indictor combination or workflow of COVID-19[17, 18] still lack large-scale clinical verification, while LDH has been widely proved to be an important marker to indicate the progress of the disease[3,4,5,6,17,18]. Our research further shows that LDH has outstanding practical predictive performance for disease progression from many aspects.
This study has some limitations. First, we did not measure viral load and some patients lacked cytokine testing, which could be factors related to the severity of the disease. Second, we did not test the LDH isoenzymes due to limited resources. LDH isoenzyme analysis in the future may help to identify the source of increased LDH.
In conclusion, LDH was found to be a superior indicator of disease status among COVID-19 patients and had the potential to optimize the clinical management strategy.