Unfractionated Heparin Safety in COVID-19: Incidence and Risks of Bleeding Complications in Japan

Abstract

Aim: Several studies have shown the efficacy and safety of low-molecular-weight heparin use in coronavirus disease 2019 (COVID-19), but that of unfractionated heparin (UFH) has not been investigated. We investigated the prevalence of bleeding complications during UFH administration, its impact on mortality, and the risk factors of bleeding outcomes associated with UFH.

Methods: This retrospective cohort study was conducted at a single-center tertiary care hospital, including hospitalized patients with COVID-19. The primary outcomes were measured as the prevalence of bleeding complications during hospitalization, and the secondary outcomes were thromboembolic events and 60-day mortality rates. Logistic regression analysis and propensity score matching were used to assess risk factors for bleeding complications and their impact on mortality.

Results: Among 1035 included patients, 516 patients were treated with UFH. Twelve (2.3%) patients in the UFH group experienced major bleeding. The prevalence of major bleeding in patients treated with therapeutic-dose UFH was 9.2%. Logistic regression analysis showed that age ≥ 60 years (adjusted odds ratio [aOR], 3.89; 95% confidence interval [CI], 1.01–15.0; P＜.05) and COVID-19 severity (aOR, 35.9; 95% CI, 4.57–282; P＜.05) were associated with major bleeding complications. After propensity score matching, 11 major and 11 non-major bleeding cases (including minor bleeding) were matched. The 60-day cumulative mortality rate between the two groups did not differ significantly (P=.13, log-rank test).

Conclusions: The incidence of major bleeding in COVID-19 patients using therapeutic-dose UFH was relatively high. Critical COVID-19 and older age were risk factors for bleeding complications.

Introduction

Severe acute respiratory syndrome coronavirus (SARS-CoV)-2 infection induces hypercoagulability and microangiopathy, which play an important role in the pathogenesis of coronavirus disease 2019 (COVID-19)¹⁾. Pathological findings in the lungs of patients with COVID-19 show endothelial cell injury and widespread microthrombi in pulmonary vessels²⁾, leading to progressive respiratory failure. Moreover, the reported prevalence of venous thromboembolism (VTE) in patients with COVID-19 is 4- to 8-fold higher than that in other respiratory infections^3-5). For these reasons, anticoagulation therapy has been deemed to improve the prognosis of patients with COVID-19.

Several studies have reported an association between anticoagulation therapy with low-molecular-weight heparin (LMWH) and decreased mortality^6-10). A previous study comparing LMWH and unfractionated heparin (UFH) for VTE prophylaxis in acutely ill medical patients showed that LMWH was more effective than UFH in reducing the risk of both deep vein thrombosis (DVT) and bleeding¹¹⁾. Despite this evidence and global usage, LMWH has only been approved for VTE prophylaxis in perioperative patients and for disseminated intravascular coagulation and extracorporeal circulation in Japan. A Japanese questionnaire-based survey revealed that 68% of patients with COVID-19 were treated with UFH, whereas only 13% of them were treated with LMWH¹²⁾. However, compared with data on the efficacy and safety of LMWH, data on UFH treatment in patients with COVID-19 are insufficient.

Aim

We conducted a retrospective cohort study with the aim of assessing the safety of UFH in the treatment of COVID-19. We also investigated the risk factors of bleeding outcomes associated with UFH.

Methods

Study Design and Populations

This single-center retrospective cohort study was conducted at the National Center for Global Health and Medicine (NCGM), a tertiary care hospital in Tokyo, Japan. The study was carried out between May 1, 2020, and September 30, 2021, and included patients who were admitted to the hospital and diagnosed with SARS-CoV-2 infection using reverse transcription-polymerase chain reaction or antigen testing. We included all patients 20 years of age or older. Although the decision for the initiation and dose of UFH was dependent on the physicians, most followed the hospital anticoagulation protocol for COVID-19 ¹³⁾. The protocol recommended UFH initiation for patients who required supplemental oxygen. We excluded patients who were taking anticoagulants, including warfarin and direct oral anticoagulants (DOACs), prior to hospitalization. The patients who were transferred from another hospital due to bleeding complications and those who were administered UFH only during direct hemoperfusion with polymyxin B immobilized fiber (PMX-DHP) were also excluded.

Data Collection

Patient data from the initial 60 days post-admission were retrospectively extracted from electronic medical charts. Furthermore, select patient data were also obtained from the COVID-19 Registry Japan (COVIREGI-JP) of the NCGM, following prior authorization. The study data were collected and managed using the Research Electronic Data Capture, a secure web-based data capture application hosted at the data center of NCGM’s Japan Clinical Research Assist Center¹⁴⁾.

The extracted data included age, sex, race, height, body weight, body mass index, performance status (Eastern Cooperative Oncology Group definition), medical history, use of anticoagulants or antiplatelet agents, vaccination record, SARS-CoV-2 variant, COVID-19 severity, pharmacotherapy, respiratory support, laboratory data, bleeding complications, thromboembolic events, and death. Laboratory data included lactate dehydrogenase, C-reactive protein, serum creatinine, estimated glomerular filtration rate (eGFR), hemoglobin (Hb), platelet count, D-dimer, international normalized ratio of prothrombin time, activated partial thromboplastin time (APTT) on admission, and maximum APTT and D-dimer levels during the study period. Patients who received a second mRNA vaccination more than 2 weeks before the onset of COVID-19 were classified as having achieved “vaccination completion.” The maximum severity of COVID-19 was determined according to the definition of the National Institutes of Health (NIH).

For UFH dosage, we extracted data on the maximum UFH dose administered within 24 hours prior to the time when the maximum APTT was recorded. We defined a prophylactic dose of UFH as 5000 international units (IU), twice daily for subcutaneous infusion, or less than 200 IU/kg/day for systemic infusion. A therapeutic dose of UFH (therapeutic-dose UFH) was defined as more than 10000 IU/day of UFH for systemic infusion, which did not meet the criteria for a prophylactic dose of UFH (prophylactic-dose UFH). Additionally, cases where the physician titrated the UFH dosage to prolong APTT, even if the dosage met the criteria of prophylactic-dose UFH, were considered as therapeutic-dose UFH.

Outcomes

The primary outcome was the prevalence of bleeding complications during hospitalization in the UFH group. We categorized bleeding complications according to the Bleeding Academic Research Consortium definition: types 1 and 2, minor bleeding; types 3 and 5, major bleeding¹⁵⁾. Type 4 bleeding was excluded because it was defined as coronary artery bypass graft-related bleeding.

The secondary outcomes were thromboembolic events, including DVT, pulmonary embolism, cerebral infarction, myocardial infarction, and mortality in the UFH group. All outcomes were assessed within 60 days of admission. The risk factors for bleeding complications, along with their impact on mortality in the UFH group, were assessed. Furthermore, the prevalence of bleeding complications and thromboembolic events in the non-UFH group was also analyzed.

Statistical Analysis

Continuous data were presented as medians with interquartile ranges, whereas categorical data were presented as numbers and percentages. We compared the background variables and mortality among patients with minor bleeding, major bleeding, and no bleeding in the UFH group using Fisher’s exact test for categorical data and the Kruskal–Wallis test for continuous data, including the APTT value. Furthermore, we compared the incidence of bleeding complications and thromboembolic events between the therapeutic and prophylactic doses in the UFH group. UFH dosage according to COVID-19 severity in the patients with bleeding complications was also analyzed. Patients with missing values were excluded from the analysis. Patients lost to follow-up were included in the analysis because UFH-related bleeding complications were believed to occur during hospitalization.

To calculate the adjusted odds ratio (OR) with a 95% confidence interval (CI) for the risk of all bleeding complications in the UFH group, a multivariable logistic regression model was employed, accounting for the following potential confounders: age, sex, history of bleeding, co-treatment with antiplatelet therapy, COVID-19 severity, UFH dosage, and eGFR and Hb levels on admission. We also explored the risk factors for major bleeding complications in the UFH group using multivariable logistic regression analysis. As there were only 12 cases of major bleeding, we decided to include two explanatory variables in this analysis. Based on previous findings, age and COVID-19 severity were chosen as explanatory variables^16-18). Because the hospital’s anticoagulation protocol for COVID-19 suggested administering a therapeutic dose of UFH to severely ill patients¹³⁾, we also performed multivariable logistic regression analysis using COVID-19 severity and UFH dosage as explanatory variables to adjust for the effect of UFH dosage.

To estimate the association between bleeding complications and mortality in the UFH group, propensity score matching was performed on each cohort to achieve a balance in covariates between patients with and without any bleeding complications (all-bleeding vs. non-bleeding) and between patients with and without major bleeding complications (major bleeding vs. non-major bleeding). We calculated the propensity scores for each cohort using multivariate logistic regression models. The included covariates were age, sex, history of bleeding, co-treatment with antiplatelet therapy, COVID-19 severity, UFH dosage, and eGFR and Hb levels on admission. Propensity score matching was performed using a 1:1 nearest-neighbor algorithm with a caliper width of 0.1. Characteristics with a standardized mean difference between cohorts of ＜0.1 were considered well-matched. No replacement was performed. The Kaplan–Meier model was used to estimate 60-day mortality, and cohorts were compared using the log-rank test. We calculated the hazard ratio (HR) with a 95% CI using the Cox proportional hazard model.

Statistical significance was set at a nominal α level of 0.05. All statistical analyses were performed using EZR (Saitama Medical Center, Jichi Medical University, Saitama, Japan), a graphical user interface for R (The R Foundation for Statistical Computing, Vienna, Austria). More precisely, it is a modified version of the R commander designed to add statistical functions frequently used in biostatistics¹⁹⁾.

Ethics Approval and Consent to Participate

This study was approved by the ethics committee of the NCGM (Approval no. NCGM-G-003612-01, approval date. 07-Sep-2020) and conducted in accordance with the Declaration of Helsinki. Patient data were anonymized prior to analysis. Informed consent was obtained in the form of an “opt-out” approach on the hospital website because of the retrospective nature of the study.

Results

A total of 1,078 patients with COVID-19 were hospitalized during the study period. Forty-three patients were excluded: 37 patients for anticoagulant use prior to admission; two patients were transferred from another hospital due to bleeding complications, and four patients received UFH only during PMX-DHP, resulting in a total of 1035 patients (519 patients in the non-UFH group and 516 patients in the UFH group) being analyzed (Supplemental Fig.1). The baseline characteristics of the patients are presented in Supplemental Table 1.

Supplemental Fig.1.

Flowchart of inclusion process

Supplemental Table 1.Baseline characteristics

		non-UFH Group n = 519	UFH Group n = 516	p value
Age, y-median [IQR]		49 [33, 65]	57 [46, 70]	＜0.001
Men, n (%)		306 (59.0)	360 (69.9)	＜0.001
Race, n (%)	Asian	508 (97.9)	504 (97.7)	0.91
	White	5 (1.0)	6 (1.2)
	Black	2 (0.4)	1 (0.2)
	Hispanic	4 (0.8)	5 (1.0)
Smoker, n (%)		244 (47.7)	230 (44.8)	0.38
BMI-median [IQR]		23.1 [20.5, 26.0]	25.6 [22.8, 28.8]	＜0.001
Comorbidities, n (%)	COPD	12 (2.3)	24 (4.7)	0.04
	Hypertension	121 (23.3)	195 (37.9)	＜0.001
	Dyslipidemia	92 (17.7)	120 (23.3)	0.03
	Diabetes mellitus	92 (17.7)	120 (23.3)	＜0.001
	Chronic kidney disease	23 (4.4)	22 (4.3)	1.00
	Hemodialysis	5 (1.0)	3 (0.6)	0.73
	Liver cirrhosis	5 (1.0)	3 (0.6)	0.73
Past history, n (%)	Ischemic heart disease	30 (5.8)	26 (5.0)	0.68
	Cerebral infraction	22 (4.2)	34 (6.6)	0.10
	Venous thromboembolism	3 (0.6)	9 (1.8)	0.09
	gastrointestinal bleeding	24 (4.6)	12 (2.3)	0.06
	Cerebral hemorrhage	16 (3.1)	11 (2.1)	0.43
Oral antiplatelet therapy, n (%)		41 (7.9)	50 (9.7)	0.32
Performance status (ECOG), n (%)	0	486 (93.6)	475 (92.1)	0.65
	1	10 (1.9)	8 (1.6)
	2	12 (2.3)	18 (3.5)
	3	5 (1.0)	5 (1.0)
	4	6 (1.2)	10 (1.9)
Vaccination, n (%)	vaccinated	7 (1.4)	8 (1.6)	1.00
	ND	117(22.5)	35(6.8)
Virus variant, n (%)	pre-VOC (ancestral)	183 (35.3)	92 (17.8)	NA
	B.1.1.7(α)	83 (16.0)	73 (14.1)
	B.1.351(β)	0 (0)	0 (0)
	P.1(γ)	0 (0)	1 (0.2)
	B.1.617.2(δ)	40 (7.7)	138 (26.7)
	α or β or γa	14(2.7)	13 (2.5)
	ND	199(38.3)	199(38.6)
Respiratory support, n (%)	no support	437 (84.2)	51 (9.9)	NA
	low flow oxygen	75 (14.5)	259 (50.2)
	high-flow nasal canula	5 (1.0)	85 (16.5)
	Noninvasive positive pressure ventilation	2 (0.4)	83 (16.1)
	Invasive positive pressure ventilation	0 (0.0)	38 (7.4)
Severity, n (%)	asymptomatic	0 (0)	0 (0)	NA
	mild	182 (35.1)	4 (0.8)
	moderate	255 (49.1)	47 (9.1)
	severe	77 (14.8)	335 (64.9)
	critical	5 (1.0)	130 (25.2)
Cotreatment, n (%)	Remdesivir	97 (18.7)	424 (82.2)	＜0.001
	Tocilizumab	4 (0.8)	103 (20.0)	＜0.001
	Baricitinib	5 (1.0)	84 (16.3)	＜0.001
	Steroids	86 (16.6)	490 (95.0)	＜0.001
	casirivimab/imdevimab	22 (4.2)	7 (1.4)	0.007
	Proton pump inhibitor	85 (16.4)	419 (81.2)	＜0.001
Laboratory data-median [IQR]	LDH on admission(U/L)	207 [168, 261]	362 [268, 500]	＜0.001
	CRP on admission(mg/dL)	1.11 [0.29, 3.89]	7.07 [3.56, 11.91]	＜0.001
	Creatinine on admission(mg/dL)	0.77 [0.61, 0.91]	0.82 [0.66, 1.03]	＜0.001
	eGFR on admission	79.0 [66.5, 94.9]	70.2 [56.1, 88.3]	＜0.001
	Hemoglobin on admission(g/dL)	14.2 [12.7, 15.4]	14.2 [13.1, 15.4]	0.17
	Platelet count on admission(×104/μL)	19.5 [16.0, 24.4]	18.3 [14.6, 23.6]	0.01
	PT-INR on admission	0.97 [0.93, 1.02]	0.98 [0.92, 1.03]	0.54
	APTT on admission(sec)	32.0 [30.0, 34.5]	34.0 [31.0, 37.0]	＜0.001
	D-dimer on admission(μg/mL)	0.0 [0.0, 0.8]	0.80 [0.0, 1.9]	＜0.001

^a N501Y-positive and E484K-positive/unknown and L452R-negative and before the importation of the omicron virus variant. Boldface values indicate P＜.05.

Table 1 shows the primary outcomes and prevalence of bleeding complications in the UFH group. The median maximum APTT of patients with bleeding outcomes was significantly longer than that of patients without bleeding outcomes (Kruskal–Wallis test with post hoc Steel–Dwass test: major bleeding vs. minor bleeding: P=.08; major bleeding vs. non-bleeding: P＜.05; minor bleeding vs. non-bleeding, P＜.05). Therapeutic-dose UFH was more frequently administered in patients with bleeding outcomes than in those without bleeding outcomes (Fisher’s exact test with post hoc Bonferroni test: major bleeding vs. minor bleeding: P=.15; major bleeding vs. non-bleeding: P＜.05; minor bleeding vs. non-bleeding: P＜.05). The incidence of major bleeding complications was 8 (9.2%) in the therapeutic-dose UFH group and 4 (0.9%) in the prophylactic-dose UFH group (Supplemental Table 2). The UFH dosage, categorized by COVID-19 severity and maximum APTT, in patients with bleeding complications is presented in Supplemental Table 3. Notably, maximum APTT was prolonged even in the prophylactic-dose UFH group.

Table 1.Incidence of bleeding complications and death and background variables

	major	minor	non-bleeding	p-value
n (%)	12 (2.3)	61 (11.8)	443 (85.9)
Age, y-median [IQR]	68 [61–73]	65 [51–75]	55 [45–70]	＜0.001
Male, n (%)	9 (75.0)	53 (63.9)	308 (69.5)	0.53
maximum APTT, median [IQR]	66.5 [56–89.8]	51.0 [41.0–72.0]	40 [35–52]	＜0.001
BW, median [IQR]	67.7 [59.9–71.9]	72.1 [60.0–80.9]	69.7 [59.3–82.1]	0.59
Hypertension	7 (58.3)	21 (35.0)	167 (37.7)	0.32
Past history of GIB	2 (16.7)	1 (1.7)	9 (2.0)	0.04
Past history of CH	0	2 (3.3)	8 (2.0)	0.71
Liver cirrhosis	0	0	3 (0.7)	1.00
Hemodialysis	0	1 (1.6)	2 (0.5)	0.37
Oral antiplatelet therapy	2 (16.7)	10 (16.7)	38 (8.6)	0.07
Therapeutic dose UFH	8 (66.7)	20 (32.8)	59 (13.3)	＜0.001
Laboratory data on admission, median [IQR]
eGFR	43.8 [26.7–71.4]	65.7 [50.3–83.1]	71.7 [57.2–89.2]	0.004
Hemoglobin	14.6 [13.3–15.9]	13.9 [12.9–14.9]	14.3 [13.1–15.4]	0.32
Platelet count	16.4 [14.8–19.8]	18.1 [14.2–22.3]	18.5 [14.6–23.8]	0.42
ECMO, n (%)	2 (16.7)	2 (3.3)	0	＜0.001
Death, n (%)	7 (58.3)	10 (16.4)	17 (3.8)	＜0.001

ECMO: extracorporeal membrane oxygenation; IQR: interquartile range; APTT: activated partial thromboplastin time; BW: body weight; UFH: unfractionated heparin; eGFR: estimated glomerular filtration rate; GIB: gastrointestinal bleeding; CH: congenital hypothyroidism.

Post hoc analysis was performed using the Bonferroni test for categorical variables and the Steel–Dwass test for continuous variables. Boldface values indicate P＜.05.

Supplemental Table 2.Incidence of bleeding complications and thromboembolic events in therapeutic and prophylactic dose UFH group

	Therapeutic dose UFH n = 87	Prophylactic dose UFH n = 427	p value
major bleeding, n (%)	8 (9.2)	4 (0.9)	＜0.001
minor bleeding, n (%)	20 (23.0)	41 (9.6)	0.048
thromboembolic event, n (%)	22 (25.0)	10 (2.3)	＜0.001

Boldface values indicate P＜.05.

Supplemental Table 3.UFH dosage according to COVID-19 severity and maximum APTT in the patients with bleeding complications

Table 3a. all cases with bleeding (n = 73)
	Therapeutic dose UFH n = 28	Prophylactic dose UFH n = 45	p value
COVID-19 severity, n
moderate	1	1	＜0.001
severe	2	27
critical	25	17
maximum APTT, median [IQR]	67 [53-89]	48 [40-66]	＜0.001

Fisher exact test with post hoc Bonferroni test: moderate illness vs. severe illness: P = 1.0; moderate illness vs. critical illness: P = 1.0; severe illness vs. critical illness, P＜.05

Table 3b. cases with major bleeding (n = 12)
	Therapeutic dose UFH n = 8	Prophylactic dose UFH n = 4	p value
COVID-19 severity, n
moderate	0	0	0.33
severe	0	1
critical	8	3
maximum APTT, median [IQR]	74 [63-90]	56 [56-67]	0.35

Boldface values indicate P＜.05.

Table 2 presents the ORs and 95% CIs, derived from logistic regression analysis, for the incidence of all-bleeding, adjusted for the previously mentioned variables. Age, COVID-19 severity, and therapeutic-dose UFH were associated with bleeding complications, with an adjusted OR of 2.1 for age ≥ 60 years (reference: ＜60 years; 95% CI, 1.17–3.78; P＜.05), 4.21 for critical illness (reference: severe illness; 95% CI, 2.36–7.50; P＜.05), and 2.09 for therapeutic-dose UFH (95% CI, 1.11–3.91; P＜.05). Age and COVID-19 severity were also associated with major bleeding complications, with an adjusted OR of 3.89 for age ≥ 60 years (reference: ＜60 years; 95% CI, 1.01–15.0; P＜.05) and 35.9 for critical illness (reference: severe illness; 95% CI, 4.57–282; P＜.05) (Table 3). COVID-19 severity was associated with major bleeding complications even after adjusting for UFH dosage (Supplemental Table 4).

Table 2.Logistic regression analysis of all-bleeding complications

Variables	Adjusted OR	95٫ CI	p value
Age ≥ 60	2.10	1.17–3.78	0.01
Sex	1.10	0.60–2.00	0.76
past history of bleeding	0.77	0.21–2.52	0.67
NIH criteria critical	4.21	2.36–7.50	＜0.001
eGFR on admission	1.23	0.60–2.53	0.56
Hb on admission	0.89	0.43–1.84	0.74
co treatment with antiplatelet therapy	1.58	0.72–3.50	0.26
Therapeutic dose UFH	2.09	1.11–3.91	0.02

eGFR: estimated glomerular filtration rate; NIH: National Institutes of Health; Hb: hemoglobin; UFH: unfractionated heparin; CI: confidence interval; OR: odds ratio.

Reference of COVID-19 severity: severe illness. Boldface values indicate P＜.05. Variance inflation factor was ＜1.5 for all variables.

Table 3.Logistic regressions analysis of major bleeding complications

Variables	Adjusted OR	95% CI	P value
Age ≥ 60	3.89	1.01–15.0	0.04
NIH criteria critical	35.9	4.57–282	＜0.001

Reference of COVID-19 severity: severe illness. Boldface values indicate P＜.05. Variance inflation factor was 1.0.

NIH: National Institutes of Health; CI: confidence interval; OR: odds ratio.

Supplemental Table 4.Logistic regression analysis of major bleeding complications using NIH severity and UFH dosage as explanatory variables

	Adjusted OR	95% CI	p value
Therapeutic dose UFH	3.85	1.06-13.9	0.04
NIH criteria critical	21.3	2.53-178	0.005

Reference of COVID-19 severity: severe illness. Boldface values indicate P＜.05.

Variance inflation factor was 1.06.

Propensity score matching was performed, and 68 bleeding cases were matched with 68 non-bleeding cases (Supplemental Table 5). Likewise, 11 major bleeding cases were matched with 11 non-major bleeding cases (Supplemental Table 6). The Kaplan–Meier curves are presented in Fig.1. The 60-day cumulative mortality rates between the two groups were not significantly different (all-bleeding vs. non-bleeding groups, P=.53; major and non-major bleeding groups, P=.13). The Cox proportional hazard model showed no significant difference in the HR for overall bleeding to death (all-bleeding vs. non-bleeding: HR, 1.32; 95% CI, 0.56–3.13; P=.53; major and non-major: HR, 3.22; 95% CI, 0.65–15.97; P=.15) (Supplemental Table 7).

Supplemental Table 5.propensity score matching result of all-bleeding and non-bleeding cohort

	non-bleeding	all-bleeding	p value	SMD
Patients, n	68	68
antiplatelet therapy, n (%))
no	60 (88.2)	58 (85.3)	0.80	0.09
yes	8 (11.8)	10 (14.7)
history of bleeding, n (%)
no	65 (95.6)	65 (95.6)	1.00	＜0.001
yes	3 (4.4)	3 (4.4)
NIH criteria, n (%)
severe	31 (45.6)	31 (45.6)	1.00	＜0.001
critical	37 (54.4)	37 (54.4)
Age ≧75
no	54 (79.4)	52 (76.5)	0.84	0.07
yes	14 (20.6)	16 (23.5)
eGFR ＜45
no	53 (77.9)	54 (79.4)	1.00	0.04
yes	15 (22.1)	14 (20.6)
Hb male ＜13, female ＜12
no	55 (80.9)	55 (80.9)	1.00	＜0.001
yes	13 (19.1)	13 (19.1)
Sex, n (%)
Female	22 (32.4)	21 (30.9)	1.00	0.03
Male	46 (67.6)	47 (69.1)
Therapeutic dose UFH, n (%)
no	42 (61.8)	44 (64.7)	0.86	0.06
yes	26 (38.2)	24 (35.3)

Supplemental Table 6.propensity score matching result of major bleeding and non-major bleeding cohort

	non-major	major	p value	SMD
Patients, n	11	11
antiplatelet therapy, n (%)
no	9 (81.8)	9 (81.8)	1.00	＜0.001
yes	2 (18.2)	2 (18.2)
history of bleeding, n (%)
no	10 (90.9)	10 (90.9)	1.00	＜0.001
yes	1 (9.1)	1 (9.1)
NIH criteria, n (%)
severe	1 (9.1)	1 (9.1)	1.00	＜0.001
critical	10 (90.9)	10 (90.9)
Age ≧55
no	0 (0.0)	0 (0.0)	1.00	＜0.001
yes	11 (100.0)	11 (100.0)
eGFR ＜45
no	6 (54.5)	6 (54.5)	1.00	＜0.001
yes	5 (45.5)	5 (45.5)
Hb male ＜13, female ＜12
no	9 (81.8)	9 (81.8)	1.00	＜0.001
yes	2 (18.2)	2 (18.2)
Sex, n (%)
female	3 (27.3)	3 (27.3)	1.00	＜0.001
male	8 (72.7)	8 (72.7)
Therapeutic dose UFH, n (%)
no	4 (36.4)	4 (36.4)	1.00	＜0.001
yes	7 (63.6)	7 (63.6)

Fig.1. Kaplan–Meier overall survival curves for patients with and without bleeding complications

Patients were categorized into two groups: those with any bleeding (all-bleeding group, A) and those without bleeding (non-bleeding group, A); and into groups with major bleeding (major bleeding group, B) and without major bleeding (non-major bleeding group, B). Comparisons between these cohorts were conducted using the log-rank test.

Supplemental Table 7.hazard ratio of 60-day mortality

cohort	HR	95% CI	p value
All bleeding	1.32	0.56-3.13	0.53
major bleeding	3.22	0.65-15.97	0.15

Supplemental Fig.2 depicts the incidence and prevalence of thromboembolic events across different categories of COVID-19 severity. In the UFH group, a total of 32 (6.2%) patients experienced thromboembolic events, compared to two (0.39%) patients in the non-UFH group.

Supplemental Fig.2.

Incidence and prevalence of thromboembolic event in each category for COVID-19 severity

Supplemental Fig.3 depicts the incidence and prevalence of all-bleeding and major bleeding events across various COVID-19 severity categories. In the non-UFH group, a total of 22 (4.2%) patients experienced minor bleeding events, and 3 (0.58%) patients encountered major bleeding events.

Supplemental Fig.3.

Prevalence of all-bleeding and major event in each category for COVID-19 severity

Discussion

In this study, we evaluated the safety of UFH in the treatment of COVID-19 and the risk factors of bleeding outcomes associated with UFH. The results revealed two important findings. First, the incidence of major bleeding complications was 2.3% and 9.2% among all patients with COVID-19 treated with UFH and therapeutic-dose UFH, respectively. Besides, the bleeding complications did not affect the 60-day mortality. Second, we found that COVID-19 severity and age were the risk factors for bleeding complications associated with the use of UFH in COVID-19.

The present study reported a 9.2% and 0.9% incidence of major bleeding in the therapeutic-dose and prophylactic-dose UFH groups, respectively. The INSPIRATION randomized clinical trial, which compared the efficacy of intermediate- and usual-dose prophylactic LMWH in patients with COVID-19 admitted to the intensive care unit, showed that the incidences of major bleeding were 2.5% and 1.4% in the usual-dose and intermediate-dose groups, respectively⁹⁾. Similarly, the REMAP-CAP, ACTIV-4a, and ATTACC investigators demonstrated in randomized control trials, which compared therapeutic dose with prophylactic dose anticoagulation for critically ill COVID-19 patients (classified as NIH severe and critical), that approximately 90% of patients received LMWH, with the incidence of major bleeding being 3.8% in the therapeutic dose group and 2.3% in the prophylactic dose group⁶⁾. The incidence of major bleeding in the therapeutic-dose UFH group appeared to be higher than that of the LMWH previously reported, although background-adjusted comparisons were not made. Because a previous study showed that the risk of bleeding complications in DVT prophylaxis was lower in LMWH than in UFH¹¹⁾, therapeutic-dose UFH possibly increased major bleeding complications. Meanwhile, the incidence of thrombotic complications was similar between the present study and previous studies that used LMWH^{6, 9)}.

Furthermore, we found that COVID-19 severity, age, and therapeutic-dose UFH were associated with all bleeding complications. Moreover, COVID-19 severity and age were associated with major bleeding complications, even after adjustment for UFH dosage. Although several bleeding risk scores, such as ORBIT and HAS-BLED, have been established for warfarin and DOACs usage, no similar bleeding risk score for UFH usage is available in clinical practice^20-22). Several previous studies suggested that aging is associated with prolonged APTT and increased major bleeding incidence, possibly due to age-related changes in coagulation factor and heparin-binding proteins^{17, 18, 23)}. In our study, the APTT levels in patients with bleeding complications tended to be prolonged, even in patients treated with prophylactic-dose UFH. A prior study revealed that, despite employing a weight-based UFH nomogram, a mere 33.8% of patients attained the therapeutic range of APTT at the initial evaluation, and 16.3% of patients underwent excessive prolongation of APTT, particularly older, female, and lower body weight patients²³⁾. Thus, aging might be one of the factors associated with poor APTT control in our study. Moreover, a previous study revealed that the incidence of bleeding complications increases with COVID-19 severity, which is also associated with vascular endothelial cell injury, coagulopathy, and disseminated intravascular coagulation^{17, 18)}. Our results are consistent with these previous studies.

To our knowledge, this is the first study to demonstrate the safety of UFH usage and risk factors for UFH-related bleeding complications in patients with COVID-19. Although the 60-day cumulative mortality rates between the bleeding and non-bleeding groups were not significantly different, the rate of major bleeding complications in patients treated with therapeutic-dose UFH appeared high in this study. A previous study indicated that therapeutic-dose anticoagulation, wherein most populations received LMWH, did not enhance survival and organ support-free days in critically ill COVID-19 patients⁶⁾. According to this evidence and the results of our study, therapeutic-dose UFH should not be used in critically ill COVID-19 patients without confirmed thrombosis events. In addition, when using UFH in older or critically ill patients with COVID-19, APTT should be monitored frequently, even in patients administered prophylactic-dose UFH.

The present study has some limitations. First, the number of patients with major bleeding was small. Although there was no significant difference in the 60-day cumulative mortality rates between the major and non-major bleeding groups, the statistical power to detect a difference may be low due to the small number of major bleeding cases. However, we compared both groups after adjusting for potential confounders, and the results seemed to be highly valid. In addition, we included only two variables in the multivariate analysis for major bleeding due to the limited number of cases. Although other risk factors may exist, consistent with previous studies, COVID-19 severity and age were found to be risk factors for major bleeding complications. Second, the retrospective nature of our study limited our ability to control for loss to follow-up. However, bleeding complications related to UFH administration usually occur during hospitalization and not during follow-up, which affected the incidence of bleeding complications in the study. Third, the majority of the patients in the present cohort were Asian. Because a previous study that examined the prolongation of activated clotting time following the administration of UFH by race showed that activated clotting time prolongation was significantly more common in Asians²⁴⁾, the incidence of bleeding complications in our present study may have been overestimated due to differences in ethnicity. The impact of racial differences in bleeding risk associated with UFH administration has not yet been determined. Fourth, due to the variation in COVID-19 severity between the UFH and non-UFH groups, it was challenging to ascertain the effectiveness of prophylactic-dose UFH in preventing DVT and enhancing mortality and morbidity outcomes. Moreover, a prior study indicated that therapeutic-dose LMWH augmented organ support-free days in severe COVID-19 patients needing low-flow oxygen support; however, this analysis did not assess UFH’s efficacy in this specific group. To establish the suitable UFH dosage for each level of severity, an interventional study is essential. Finally, we could not conduct a direct comparison between UFH and LMWH because LMWH is rarely used in COVID-19 in Japan. Further investigations are warranted to determine the difference in efficacy and safety between UFH and LMWH in COVID-19 patients.

Conclusion

Our study showed that the incidence of major bleeding complications in patients with COVID-19 using therapeutic-dose UFH was relatively high, although bleeding complications did not affect 60-day mortality. In addition, critical COVID-19 and age ≥ 60 years were risk factors for major bleeding complications associated with UFH administration in COVID-19.

Acknowledgements

We thank all the clinical staff at our hospital for their dedication to clinical practice and patient care.

Notice of Grant Support:

This work was supported by Sysmex Co., Ltd, through a collaborative research agreement with the Lubna Sato and National Center for Global Health and Medicine Intramural Research Fund (grant number F20C02101) with Noriko Iwamoto.

Conflicts of Interest

All authors declare no conflicts of interest regarding this study.

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