Reports of blood clots in COVID-19 patients are “increasingly prominent,” even in those who are given drugs to prevent them, said the authors of the research published in the journal The Lancet. After they noticed this pattern in patients at their institution, the team set out to find an explanation.

The study involved 15 critically ill COVID-19 patients in an Emory University intensive care unit. All of the patients were given drugs to prevent blood clots, but the viscosity of their blood plasma was 95 percent higher than normal, the team said. Four of the patients had blood clot-related compilations, including a pulmonary embolism, restricted blood supply in a limb and clotting related to a kidney support machine. The researchers also found a strong correlation between plasma viscosity and a measure of how seriously ill a patient is called the Sequential Organ Failure Assessment score.