Patients with abnormally high blood sugar levels are more than twice as likely to die from COVID-19, researchers in China said on Saturday.

It is the first time scientists have been able to confirm that patients with hyperglycemia, but not diagnosed with diabetes, are at higher risk of death from COVID-19, they wrote in the journal Diabetologia.

The researchers examined death rates for 605 COVID-19 patients at two hospitals in Wuhan, China.

Having high blood pressure is "independently associated" with increased risk of death and complications from COVID-19, they wrote.

The study builds on previous research on diabetic patients.

One-in-10 COVID-19 patients with diabetes died in French hospitals, a far higher proportion than for patients without the condition, a May study in the same journal found.

Exactly why high blood sugar increases COVID-19 death rates remains unclear.

The authors of Friday's study suggested that blood clotting, the weakening of blood vessel linings, and cytokine storm syndrome – an overreaction of the immune system – could all play a role.

The authors of the report urged hospitals to test all COVID-19 patients for glucose levels, as opposed to only those known to have diabetes.

The study, which looked at hospital patients admitted in January and February, had some limitations, experts not involved in the research said.

"This is a nice report but it is fully in line with expectations," Naveed Sattar, a professor of metabolic medicine at the University of Glasgow who was not one of the study's authors, said.

"What the authors cannot confirm is whether differential targeting of blood sugar levels in those admitted leads to differences in outcomes."

More research involving randomized controlled trials is needed, Bernard Khoo, a professor of endocrinology at University College London, commented.