Theranos may be about to lose its partnership with the largest drug-retailing chain in the US.

The Wall Street Journal is reporting that Walgreens, which operates more than 40 Theranos Wellness Centers --places where people can have their blood tested--out of its stores in Arizona and California, sent the blood-testing company a warning letter in late January that gave it 30 days to resolve the problems found with its lab in California or the partnership would be terminated.

Last month, the Centers for Medicare and Medicaid Services sent a letter saying it found problems with Theranos' northern California lab, saying some of its practices "pose immediate jeopardy to patient health and safety."

Other possible violations included analytic systems and three instances of laboratory-personnel problems. Theranos has until Friday to respond to the agency, which has oversight over clinical laboratories.

Most of Theranos' blood tests are run out of its Arizona lab, which was not involved in the CMS warning. Theranos says it runs about 95% of all Walgreens wellness center lab tests out of its Arizona lab.

The wellness centers in Arizona were part of a pilot program between Walgreens and Theranos, and Walgreens has not said it has any immediate plans to expand that partnership. Theranos has also partnered with a healthcare company in Pennsylvania, which has suspended Theranos tests until the issues cited in the CMS warning are resolved.

Should this partnership dissolve, Theranos is at risk of losing the majority of its retail blood-testing space.

Walgreens declined to comment. We've reached out to Theranos for comment as well and will update if we hear back.

This story first appeared on Business Insider.