Laboratory tests capable of discriminating between severe and non-severe cases will allow better risk stratification and appropriate allocation of resources.
Almost 5.7 million cases of COVID-19 have been confirmed worldwide, resulting in 355.653 deaths. For such a high volume of patients presenting at hospitals and other health care centers, accurate diagnosis and efficient markers for disease severity, therapeutic response, and disease outcome are key. Laboratory test biomarkers can play a useful role here.
The current standard for diagnosis of infection with SARS-CoV-2 is rRT-PCR. Blood gas analysis is valuable in diagnosis and predicting clinical worsening in COVID-19 patients, including in a patient subset that does not exhibit any obvious respiratory difficulties (‘happy hypozia’). In addition, identification of new laboratory tests capable of discriminating between severe and non-severe cases or patients at high or low risk of mortality will allow better risk stratification and appropriate allocation of resources [1, 2]. Mortality among patients with severe COVID-19 could be significantly reduced by early identification and timely treatment of critical cases . In some countries, biomarkers such as CRP, PCT, lymphopenia, and IL-6 and IL-10 are already being used to aid diagnosis or to provide early evidence of more severe disease progression .
Essential laboratory parameters
Routine laboratory examinations reveal a number of abnormalities in COVID-19 patients. A meta-analysis of 19 observational studies involving almost 3,000 patients with confirmed COVID-19 found that the most common laboratory features reported were decreased serum albumin (76% prevalence), elevated CRP (58%), elevated LDH (57%), lymphopenia (43%), and high ESR (42%) . Another review of eight smaller studies reported similar abnormalities in COVID-19 patients, i.e. lymphopenia (35-75%), elevated CRP (75-93%), LDH (27-92%), and ESR (up to 85% of cases) and low concentrations of serum albumin (50-98%), as well as increased D-dimer (36-43%) and low hemoglobin (41-50%) . Other abnormalities reported were increases in neutropenia, total bilirubin, creatinine, cardiac troponin, PT, and PCT .
Disease severity and risk stratification
Lymphopenia, neutrophilia, elevated serum ALT and AST levels, elevated LDH, CRP, and ferritin levels have been associated with greater disease severity [8, 9] and hospitalization  in COVID-19 patients. Patients with severe and fatal disease have significantly increased white blood cell (WBC) count, and decreased lymphocyte and platelet counts compared to those with non-severe disease and survivors [11,12]. CRP levels are markedly elevated in all patients, but significantly higher in severe than in non-severe cases. The elevated white blood cell and CRP levels in severe COVID-19 patients may reflect accompanying bacterial infection.
Elevated ALT, AST, and creatinine seen in severe cases suggests that COVID-19 carries an increased risk of impaired liver and kidney function. Biomarkers of inflammation such as IL-6 and IL-10, of cardiac injury, of liver and kidney function, and of coagulation measures are also significantly elevated in patients with both severe and fatal COVID-19. A recent meta-analysis of data from 21 studies concluded that WBC count, lymphocyte count, platelet count, IL-6, and serum ferritin should be closely monitor as markers for potential progression to critical illness .
A retrospective comparison of the hematological parameters between mild and severe cases showed that IL-6 and D-dimer were closely related to the occurrence of severe COVID-19 in adults, and their combined detection had the highest specificity and sensitivity for early prediction of the severity of COVID-19 . A similar result was reported from a pooled analysis of nine studies involving 1,779 COVID-19 patients, in which low platelet count was associated with increased risk of severe disease and mortality in patients and served as a clinical indicator of worsening illness during hospitalization, the researchers said .
Elevated D-dimer and lymphopenia have been associated with mortality in several studies [14,15]. Among a cohort of 799 COVID-19 patients, concentrations of ALT, AST, creatinine, CK, LDH, cardiac troponin I, N-terminal pro-brain natriuretic peptide, and D-dimer were markedly higher in patients who died compared with those who recovered .
Correlation between abnormal coagulation parameters and disease severity
Abnormal coagulation parameters are associated with poor prognosis in COVID-19 patients. D-dimer is commonly elevated in patients with COVID-19, especially in older individuals and those with comorbidities who are at higher risk of dying from COVID-19. D-dimer level correlates with disease severity [17, 18] and is a reliable prognostic marker for in-hospital mortality [17,19]. Higher D-dimer levels on admission are linked to patients needing critical care support .
In a large analysis of data from 1,099 patients with laboratory-confirmed COVID-19 from over 550 hospitals in China, raised D-dimer (≥0.5 mg/L) was recorded in 46% of patients, 43% with non-severe, but 60% with severe illness . PT was slightly prolonged in non-survivors at admission versus survivors and in those who needed critical care support versus the non-ICU cohort.
The International Society of Thrombosis and Haemostasis (ISTH) recommends measurement of D-dimers, prothrombin time and platelet count in all patients with COVID-19 infection . Patients who have markedly raised D-dimers (3-4 fold increase), should be admitted to hospital even in the absence of other severity symptoms. In addition, the ISTH suggests that serum fibrinogen measurement may be valuable for diagnosis of DIC .
Measuring inflammatory markers
Patients with critical illness have high plasma levels of inflammatory markers, suggesting potential immune dysregulation. [22, 24] Higher serum levels of pro-inflammatory cytokines (TNFα, IL-1, and IL-6) and chemokines (IL-8) were found in patients with severe COVID-19 compared with individuals with mild disease, similar to results seen in SARS and MERS and suggest a role for hyperinflammatory responses in COVID-19 pathogenesis . This excessive immune response, called a cytokine storm, arises from overproduction of early response proinflammatory cytokines such as TNF, IL-6, and IL-1β and can lead to ARDS, multi-organ failure, and, ultimately, death.
UK researchers recommend that all patients with severe COVID-19 should be screened for hyperinflammation using laboratory trends (e.g. increasing ferritin, decreasing platelet counts, or erythrocyte sedimentation rate) to identify patients that can be treated with immunosuppressive therapies such as steroids, intravenous immunoglobulin, selective cytokine blockade or JAK inhibition . Based on a study of 4,000 patients hospitalized in New York City, in whom D-dimer, ferritin, and CRP were strongly associated with critical illness, NYU Grossman School of Medicine researchers recommend routine measurement of inflammatory markers during COVID-19 hospitalization .
PCT is typically normal on admission, as in other viral infections, but it may increase in COVID-19 patients admitted to ICU [3, 20, 26]. In a pooled analysis of four studies, increased PCT was associated with an almost five-fold higher risk of severe SARS-CoV-2 infection . This may represent a bacterial coinfection in these patients, but it could also be a marker of severity of ARDS or the result of immune dysregulation that increases the production of cytokines that increase procalcitonin synthesis .
CK and LDH
COVID-19 mRNA clearance ratio has been identified as significantly correlated with a fall in serum CK and LDH levels that could predict a favorable outcome in COVID-19 infected patients . In recently published studies, myalgia or fatigue affected 44-70% of hospitalized patients and increased CK was present in up to 33% of admitted patients . Other coronavirus infections are associated with myalgias and elevated CK and rhabdomyolysis, suggesting that coronavirus infections may cause viral myositis.
Cardiac and kidney injury markers
COVID-19 infections are associated with increased levels of cardiac biomarkers due to myocardial injury probably associated with infection-induced myocarditis and ischemia . In multivariable adjusted models, cardiac injury is significantly and independently associated with mortality. Similarly, elevated troponin levels due to cardiac injury are associated with significantly higher mortality . Severe COVID-19 infections are also potentially associated with cardiac arrhythmias at least in part due to infection-related myocarditis.
It is not yet known whether the kidneys are a major target of the virus. [33, 34]. Renal impairment has been reported to be common in COVID-19 patients , and AKI frequently develops during hospitalization for COVID-19 and is associated with in-hospital mortality. Monitoring kidney function in COVID-19 is recommended especially in patients with elevated plasma creatinine . A small study of 59 patients infected with SARS-CoV-2 reported  that 63% of patients exhibited proteinuria, and 19% and 27% respectively had elevated plasma creatinine and blood urea nitrogen. Based on these findings it was concluded that laboratory criteria such as levels of IL-6 and other cytokines as well as cell cycle arrest biomarkers with high predictive value for AKI could represent objective and standardized criteria to guide therapy.
More data needed
No biomarker or combination of biomarkers currently exists that is sensitive or specific enough to establish a diagnosis of COVID-19, or to pragmatically predict its clinical course. To date, most of the relevant data on COVID-19 has come from case series or observational studies. As more data become available and more prospective studies are reported, laboratory parameters may emerge that can discriminate between severe and non-severe cases, or patients at high or low risk of mortality.
About the Author
Linda Brookes is a medical writer and editor who divides her time between London and New York, working for a variety of clients in the healthcare and pharmaceutical fields.
List of Abbreviations
AKI: acute kidney injury
ALT: alanine aminotransferase
ARDS: acute respiratory distress syndrome
AST: aspartate aminotransferase
CEBM: Centre for Evidence-Based Medicine (UK)
CK: creatine kinase
COVID-19: novel coronavirus disease 2019
CRP: C-reactive protein
DIC: disseminated intravascular coagulation
ESR: erythrocyte sedimentation rate
ICU: intensive care unit
IL-6: interleukin 6
IL-8: interleukin 8
IL-10: interleukin 10
LDH: lactate dehydrogenase
MERS: Middle Eastern respiratory syndrome
PT: prothrombin time
rRT-PCR: real-time reverse transcription polymerase chain reaction
SARS: severe acute respiratory syndrome
SARS-CoV-2: severe acute respiratory syndrome coronavirus 2
TNFα: tumor necrosis factor α