– High-dose blood thinners result in little or no difference in death rate and increase minor bleeding compared to low-dose blood thinners for people hospitalised with COVID-19. Giving blood thinners compared to not giving blood thinners might reduce the death rate.
– It is very likely that new studies will not change the evidence about the effects of different doses of blood thinners on death rate and minor bleeding. High-quality studies are still needed to analyse the need for additional respiratory support, giving blood thinners compared to no blood thinners, comparing different blood thinners, and giving blood thinners for extended periods.
What is COVID-19?
COVID-19 typically affects the lungs and airways; however, in addition to respiratory problems, about 16% of people hospitalised with COVID-19 experience problems with their blood vessels, leading to blood clots forming in the arteries, veins and lungs. Nearly half of all people with severe COVID-19 in intensive care units develop clots in their veins or arteries.
What are blood thinners?
Blood thinners are medicines that prevent harmful blood clots from forming (deep vein thrombosis). However, they can cause unwanted effects such as bleeding. Some guidelines recommend giving blood thinners when people are first admitted to hospital with COVID-19 to prevent blood clots from developing, rather than waiting to see whether blood clots develop and then treating them with blood thinners.
What did we want to find out?
We wanted to know whether giving blood thinners to people hospitalised with COVID-19 as a preventive measure reduced the number of deaths compared to people who received no treatment or those who received a placebo treatment (an identical-seeming treatment but with no active ingredient). We also wanted to determine whether these individuals needed less support with breathing, whether they still developed harmful blood clots, whether they experienced bleeding and whether they experienced any other unwanted events.
What did we do?
We searched for studies that assessed blood thinners given to people hospitalised with COVID-19 to prevent blood clots. Studies could be of any design as long as they compared a blood thinner with another blood thinner, no treatment or a placebo. Studies could take place anywhere in the world and participants could be any age as long as they were in hospital with confirmed COVID-19 disease. We pooled the results when appropriate.
What did we find?
We included seven studies with 16,185 people hospitalised with COVID-19 in either intensive care units, hospital wards or emergency departments. Studies were from Brazil (2), Iran (1), Italy (1), and the USA (1), and two involved more than country. People in the studies were aged from 55 to 68 years on average. Studies lasted from 15 to 90 days and provided evidence on deaths, bleeding, blood clotting, length of hospital stay and unwanted effects. There was little or no evidence on need for respiratory support (help with breathing), deaths related to COVID-19, and quality of life.
Higher-dose of blood thinners compared with lower-dose (4 studies, 4647 people) In people who received higher compared to lower doses of blood thinners there was little to no difference in death rate. However, people on higher doses were more likely to experience minor bleeding compared to in those on lower doses. People who received higher doses of blood thinners likely had reduced pulmonary embolism (blood clot in the lung or blood vessel leading to the lung), slightly increased major (more severe) bleeding, and probably had little to no difference in time spent in hospital compared to those who received the lower doses of blood thinners. In people who received higher doses of blood thinners, there was little to no difference in the rate of deep vein thrombosis, and other unwanted events compared to those who received the lower dose of blood thinners.
Blood thinners compared with no treatment (3 studies, 11,538 people) People who received blood thinners had a reduced death rate compared to those who did not receive blood thinners, but the evidence is very uncertain.
What are the limitations of the evidence?
We are very confident that higher doses of blood thinners do not change the risk of death but do increase the risk of bleeding in people hospitalised with COVID-19.
Although our confidence in the evidence is very limited, people who receive blood thinners may have a lower death rate compared to those who did not receive any blood thinners.
What happens next?
Our searches found 62 ongoing studies with 35,470 people. We plan to add the results of these studies to our review when they are published.
How up to date is this evidence?
The evidence is up to date to 14 April 2021.