High FGF23 Levels Could Warn Doctors About Kidney Trouble in COVID-19 Patients

When someone gets really sick with COVID-19, their kidneys can start to fail. Doctors call this acute kidney injury, or AKI for short. It happens when the kidneys suddenly stop cleaning the blood the way they should. AKI is a big problem because people who get it are much more likely to die. But here is the tricky part. There are not many good ways for doctors to spot AKI early. So researchers have been looking for clues in the blood that could warn them before things get worse.

One of those clues is a protein called fibroblast growth factor 23, or FGF23. This protein is normally made by bones. It helps control how the body uses minerals like phosphate. When the kidneys are not working right, FGF23 levels can shoot way up. A team of scientists decided to test whether high FGF23 could be a red flag in COVID-19 patients.

They studied 111 people who were in the hospital with COVID-19. The patients were treated at the Indiana University Academic Health Center between April and October of 2020. The researchers measured FGF23 in their blood. They also measured a partner protein called Klotho, which helps FGF23 do its job. Then they watched the patients over time. The average follow-up was almost two years, around 22.6 months.

The results were pretty striking. Patients without kidney problems had a median FGF23 level of about 120.7 RU per mL. Those with AKI had a much higher level of 305.6 RU per mL. And patients who were already on dialysis because their kidneys had completely failed had sky-high levels, around 3,607 RU per mL. That is a huge jump. Patients with elevated FGF23 were more than twice as likely to have AKI. For every step up in the log of FGF23, the risk of death went up by about 45 percent. That was even after accounting for age, heart disease, and body weight.

Klotho told a different story. Its levels did drop in patients on dialysis, but it did not seem tied to AKI in the other patients. So Klotho may not be as useful a warning sign in this setting. The big takeaway is that FGF23 might give doctors an early heads-up. If a COVID-19 patient has rising FGF23, it could mean their kidneys are in danger. Catching that early could help doctors act faster and maybe save lives.