The Department of Intensive Care Medicine of Hospital Joan XXIII (Tarragona, Spain) carried out this project in April 2020 with a working group of four clinical investigators, with the aim of reviewing the scientific evidence and of developing recommendations of particular interest for the daily clinical management of patients with COVID-19 disease.

The different literature sources were reviewed by two investigators on an independent basis. For the drawing of conclusions, a search was made of articles published from December 2019 to 23 April 2020 in the following databases: Medline (PubMed), Cochrane Library and ScienceDirect. The keywords used individually or in combination for the search were «COVID-19», «coronavirus», «D-dimer», «disseminated intravascular coagulation» and «anticoagulation».

With regard to the inclusion criteria, and considering the current lack of knowledge about this serious new infectious disease, we reviewed meta-analyses, observational studies, review articles and clinical guides referred to adult patients hospitalized due to COVID-19 disease. Assessment of the quality of evidence was based only on the original articles.

With regard to the exclusion criteria, we excluded studies of pediatric patients, articles published in languages other than English or Spanish, and studies in animals.

Information was extracted from the publications referred to study design and period, clinical variables, statistical analysis, risk factors and possible bias. Lastly, all the articles were reviewed by the other two clinicians of the working group, with extensive research experience.

We established four questions of clinical interest in PICO format for the drawing of conclusions, and quality of evidence was assessed based on the Grading of Recommendations Assessment, Development and Evaluation (GRADE) system9. In the event of disagreement, consensus was reached among all the working group members.

Conclusion 1: D-dimer in patients with COVID-19 is associated to increased severity, progression of the disease, acute respiratory distress syndrome (ARDS) and death (quality of evidence: low).

Recommendation 1: It is advisable to monitor the D-dimer levels upon admission and every 24–48 h as a tool for evaluation of the prognosis and progression of the disease (strength of recommendation: weakly in favor).

We are increasingly learning more about the relatively frequent coagulation disorders seen in patients with COVID-19, in particular in the more serious cases. Infection due to SARS-CoV-2 appears to induce a blood hypercoagulability state, since there have been reports of coagulation disorders and elevated DD in a large proportion of patients10, with gradual increments related to progression of the disease11. All this could be explained by excessive coagulation cascade and platelet activation - with the consequent formation of intraalveolar fibrin deposits (or systematic fibrin microthrombi). These findings are more characteristic of patients with COVID-19 and ARDS. This is due to the prothrombotic response, which attempts to avoid diffuse alveolar damage and prevent the infectious agent from penetrating into the bloodstream. However, this may give rise to the formation of pulmonary microthrombi, with deleterious effects upon the patient course12. Nevertheless, certain discrepancies may arise from the multiple and sometimes opposite actions of thrombosis on the pulmonary epithelium following sepsis, since on one hand mild lung thrombosis favors repair of the damaged endothelium, while on the other severe thrombosis causes hypoxia and produces pulmonary endothelial damage13.

D-dimer is a fibrin degradation product generated from three reactions: the conversion of fibrinogen into fibrin mediated by thrombin, fibrin reticulation mediated by activated factor XIII, and fibrin degradation mediated by plasmin14. This means that the levels depend on both coagulation and the activation of fibrinolysis. D-dimer has high sensitivity in the presence of thromboembolic disease, but specificity is poor, since DD is also elevated in other situations. Sepsis (in the same way as ARDS)15 is characterized by marked inhibition of fibrinolysis; the DD levels in septic patients therefore probably do not adequately reflect the degree of fibrin formation16,17. In this respect, it may be suggested that the isolated use of DD for the diagnosis of DIC may lead to error. Likewise, DD has been previously studied, with the observation of a high prevalence of elevated DD levels in cases of community-acquired pneumonia18, severe sepsis or septic shock19 - which moreover evidences its role as a predictor of mortality in sepsis20.

Conclusion 2: Few studies report on the incidence of DIC according to the criteria of the International Society on Thrombosis and Hemostasis (ISTH), and there is little evidence on whether its presence is associated to increased mortality (quality of evidence: low).

Recommendation 2: Daily monitoring of the coagulation parameters and of the development of thrombotic or hemorrhagic manifestations is advised for the early diagnosis of DIC according to the criteria of the ISTH (strength of recommendation: weakly in favor).

According to the Scientific and Standardization Committee of the ISTH, DIC is defined as an acquired syndrome characterized by intravascular activation of the coagulation systems on a systemic basis, with thrombotic or hemorrhagic phenomena, associated to characteristic laboratory test parameter alterations and accompanied by the development of organ dysfunction as an expression of coagulation activation37 (Table 3). Disseminated intravascular coagulation has been shown to be a predictor of mortality in patients with severe sepsis and septic shock38.

Some studies offer discordant results in relation to the prevalence of DIC and its association to the prognosis of patients with COVID-19 (Table 4). Tang et al.4 found that a large proportion of deceased patients met the international criteria of DIC according to the ISTH (71.4% versus 0.6% of the survivors). This was a low level of evidence study, since the analysis was carried out when many of the patients were still admitted - with no data being reported on the existence of multiorgan failure, sepsis, ARDS or clinical manifestations of DIC (thrombosis or hemorrhage). On the other hand, Lodigiani et al.27 reported a far lower incidence of DIC according to the ISTH criteria (2.2%), with a mortality rate of 88% (n = 7). Other authors26,34,39 have reported incidences of between 6.4–22%, though without specifying the criterion used for establishing the diagnosis. In contrast, Guan et al.28, in their study of 1099 patients, including 173 with severe disease, only reported one case of DIC (incidence 0.1%), likewise without specifying the criterion used for establishing the diagnosis. Some of these studies4,26,39 coincide in reporting a higher incidence of DIC among the patients that died, though without adjusting for confounding factors.

It is therefore possible that DIC may be overdiagnosed, since most of the publications describe the alteration of isolated laboratory parameters as evidence of coagulopathy associated to the disease, without strictly complying with the diagnosis of DIC. Lippi et al.40 published a meta-analysis of 9 studies involving 1779 patients with COVID-19, including 399 with severe disease (22.4%). The analysis showed the platelet count to be significantly lower in the more seriously ill patients, and even lower in those that died. In the four studies (n = 1427) affording data on the incidence of thrombocytopenia, the latter was seen to be associated to a 5-fold higher risk of severe COVID-19 disease (OR 5.1; 95%CI: 1.8–14.6), without referring to other data suggestive of DIC. In relation to the coagulation times, Huang et al.23 recorded longer prothrombin times in critical patients. However, two other studies22,24 reported DD and fibrinogen levels in the more seriously ill patients, though without evidencing alterations in coagulation time. Analyses have also been made of coagulation anomalies based on traditional tests and thromboelastometry profiles in a group of 22 cases admitted to the ICU due to COVID-19 versus healthy controls5. The cases presented significantly higher DD and fibrinogen levels than the controls (p < 0.0001). Furthermore, thromboelastometry profiles evidencing hypercoagulability were recorded, reflected by shorter clot forming times and greater maximum clot firmness values (p < 0.001). It therefore was concluded that patients with COVID-19 presenting hyperfibrinogenemia (resulting in increased fibrin formation and polymerization, which may predispose to thrombosis) present severe hypercoagulability instead of consumption coagulopathy as in the context of DIC.

Likewise, as reported by the American Society of Hematology41, in contrast to the pattern seen in classical DIC secondary to bacterial sepsis or trauma, the coagulopathy observed in patients with COVID-19 is characterized by the elevation of fibrinogen and DD, which is correlated to a parallel increase in inflammatory markers, and prolongation of prothrombin time and activated partial thromboplastin time (aPTT), while thrombocytopenia, if seen, is usually mild to moderate. Furthermore, in both sepsis and ARDS, we observe an increase in procoagulating activity, with pulmonary vascular microthrombosis (immunothrombosis) and a decrease in fibrinolytic activity that contributes to fibrin formation due to endothelial dysfunction following the excessive proinflammatory response to the viral infection. These pulmonary fibrin microthrombi have been found both in the presence and in the absence of DIC15. Therefore, it is possible that the laboratory test findings in patients with COVID-19, such as the increase in degradation products of fibrinogen/DD, should not always be attributed to DIC.

In consequence, coagulopathy associated to COVID-19 disease appears to be associated with a hypercoagulability profile different from that of consumption coagulopathy. Some patients with severe SARS-CoV-2 infection may develop coagulopathy meeting DIC criteria according to the ISTH, with the fulminant activation of coagulation and the consumption of coagulation factors, moderate to severe thrombocytopenia, the prolongation of prothrombin time and activated partial thromboplastin time, marked DD elevation and decreased fibrinogen. However, DIC involves a complex clinical and laboratory test diagnosis that cannot be established only from the isolated laboratory test data42. Consequently, based on the data available at this time, it is not possible to establish its incidence or association to the prognosis of patients with SARS-CoV-2 pneumonia.

Conclusion 3: There is no evidence that empirical anticoagulation at full or intermediate doses results in improved outcomes in patients with COVID-19 and elevated DD (quality of evidence: none).

Recommendation 3: It is not advisable to prescribe empirical anticoagulation in patients with COVID-19 according to the DD levels. Such treatment should only be administered in the context of a controlled clinical trial (strength of recommendation: strongly against).

In coagulopathy, and independently of its cause, treatment of the underlying condition is essential. In the case of COVID-19 infection, given the lack of a specific treatment shown to afford benefit3, supportive care is currently the only option for improving the course of the disease. The recent literature indicates that COVID-19 may predispose to the development of arterial and venous thromboembolic complications as a consequence of the excessive inflammation, hypoxia, patient immobilization and the possible development of DIC. Klok et al.7, in a series of 184 critical patients with SARS-CoV-2 pneumonia, reported a cumulative incidence of such complications of 31%, including venous thromboembolism and arterial thrombosis.

It has been reported that microvascular thrombosis is implicated in hypoxemic respiratory failure in some patients with COVID-19. The necropsy studies to date are limited, but some point to the existence of microvascular thrombosis in the pulmonary circulation43,44. In consequence, it has been postulated that benefit may be obtained from the administration of anticoagulation in the management of critical patients with high DD levels or altered coagulation parameters (coagulopathy or DIC)8,45. A clear example of this is provided by the recommendations of the Spanish Society of Intensive and Critical Care Medicine and Coronary Units (Sociedad Española de Medicina Intensiva, Crítica y Unidades Coronarias [SEMICYUC])46, which include the consideration of anticoagulation in patients with elevated DD (>2000 ng/mL). Similarly, the Cardiovascular Thrombosis working group of the Spanish Society of Cardiology (Sociedad Española de Cardiología)47 has developed a consensus document based on author opinions or small case series, recommending anticoagulation in patients with severe COVID-19 and a high thromboembolic risk – the latter being defined as high DD levels or elevated proinflammatory markers, among other parameters.

However, there is presently no scientific evidence to support such treatment. In fact, our literature search only identified the study published by Tang et al.48, involving 449 patients, which compared patients who received heparin (7 days of low molecular weight heparin or unfractionated heparin) versus those who did not. The mortality rate was 29.8%, with no differences being observed after 28 days between the heparin and non-heparin groups (30.2% versus 29.7%). The authors used the sepsis-induced coagulopathy score (SIC)49 instead of the DIC score of the ISTH37. With this classification, 21.6% of the patients met criteria for an SIC score ≥4, and in these subjects the administration of heparin was associated to lesser mortality (40.0% versus 64.2%; p = 0.03), though not so in the patients with an SIC score <4. Similarly, in the patients with DD >3 μg/mL (6 times the upper limit of normal), the administration of heparin was associated to a 20% decrease in mortality rate. However, this study had important limitations, since no analysis was made of the use of anticoagulation therapy; instead, it compared the use of heparin as prophylaxis (use recommended in hospitalized patients, and regarded as good clinical practice) versus no such use (malpractice). Likewise, this was a retrospective study, with possible selection bias, that did not report the characteristics of the compared groups regarding patient severity, and no multivariate analysis was made to assess SIC and mortality. In view of the above, these results must be viewed with great caution.

According to most of the studies published to date50, there is no indication for full dose empirical anticoagulation in patients with COVID-19 disease, unless clinical thrombosis or thromboembolism has been documented51 or there is some other classical indication for its use (mechanical valves, atrial fibrillation, etc.). In fact, to date there is no published evidence justifying an increase in heparin dose in patients with severe COVID-19 disease; such practice therefore should only be applied in the context of controlled clinical trials. Doing otherwise would be regarded as clinical malpractice. In effect, new treatments must be evaluated in randomized controlled trials to truly understand both their benefits and their associated risks52. Many of the failed hypotheses in clinical research over the last 30 years have reemerged with the hope of affording new therapies for COVID-19. Maintaining the principles of evidence based medicine in critical patient care, as has been demonstrated in randomized multicenter trials, will improve the outcomes of patients with severe COVID-19 disease.

Similarly, many institutional protocols, including those of the Spanish Society of Hematology (Sociedad Española de Hematología)53, have advocated intermediate intensity thromboprophylaxis (i.e., the standard daily prophylactic dose of low molecular weight heparin twice a day), and have proposed its use in the case of patients with a high risk of thrombosis7, even in the absence of supporting scientific evidence. In fact, both the World Health Organization (WHO)54 and different societies50 continue to recommend standard pharmacological thromboprophylaxis doses.

It is clear that thromboprophylaxis should be administered to all patients admitted to hospital, in accordance with the current clinical practice guides51,55. In the case of patients hospitalized with COVID-19, with an increased risk of thrombosis because of their condition, and due to the procoagulant state associated to the disease, the current recommendation to use heparin at standard prophylactic doses (daily low molecular weight heparin corrected for body weight and renal clearance or fondaparinux, proposed in preference of unfractionated heparin in order to reduce contact) should also be maintained in order to prevent thrombotic events46,56. A panel of medical experts from China and Europe have developed a consensus document based on the evidence regarding the prevention and management of thromboembolic disease associated to COVID-19 that confirms this57. Thromboprophylaxis should be maintained, despite anomalous coagulation test results, in the absence of active bleeding, and it should only be suspended if the platelet count drops to below 25–30 × 109/l. Mechanical thromboprophylaxis is to be used when pharmacological thromboprophylaxis is contraindicated41,46.

Conclusion 4: There is no evidence to justify the use of anticoagulation in DIC associated to COVID-19 (quality of evidence: none).

Recommendation: It is not possible to recommend the administration of anticoagulation therapy in patients with DIC associated to COVID-19, except in cases with confirmed thrombotic phenomena (strength of recommendation: weakly against).

In the case of confirmed DIC associated to SARS-CoV2 disease, and in the same way as with DIC of any other cause, the effectiveness of anticoagulation is subject to controversy, despite the conduction of multiple randomized controlled trials49. Resolution of the triggering event or underlying disease is crucial for management of the syndrome, without anticoagulation therapy having been shown to improve the prognosis58. Consequently, and in the absence of any other indication for anticoagulation (documented ischemic phenomena or purpura fulminans), these patients should only receive thromboprophylaxis50.