Blood tests by startup Theranos Inc. gave irregular results more often than testing services offered by large laboratories such as Quest Diagnostics Inc. and Laboratory Corp of America Holdings, according to the first independent study published on Theranos’s tests.

When tested on 60 patients, Theranos’s finger-prick technology also rejected samples 2.2% of the time, compared to virtually never in other services, according to the study by researchers at the Icahn School of Medicine at Mount Sinai in New York, published Monday in the Journal of Clinical Investigation. The patients were tested on 22 measures, including for cholesterol.

Theranos’s results were outside of the normal range more often than its peers, at 12.2% compared to 8.3% for LabCorp and 7.5% for Quest, according to the study. Still, when comparing test data between services, Theranos’s mostly agree with other services, with the exception of cholesterol measures, which were consistently low, the researchers wrote.

Theranos has “significant concerns regarding the study’s methods and conclusions,” the startup’s lab directors wrote in a letter to the Journal. The rejection rate isn’t reliable because collecting finger samples after drawing blood from a person’s veins can skew the results, Daniel Young, Theranos’s lab director in Arizona, said in a telephone interview.

The study will add to the scrutiny on closely held Theranos, founded by entrepreneur Elizabeth Holmes, which has been under fire in recent months as critics have questioned the accuracy of its technology. The company has struggled to regain credibility ever since the Wall Street Journal first published a series of articles questioning its tests.

Drops of Blood

The Palo Alto, California-based company has been valued in private investments at about $9 billion after promising a technology capable of performing lab tests more cheaply than rivals, in many cases using just drops of blood from a patient’s finger. In October, Theranos said it’s using its proprietary nanotainer collection-tube technology in only one of about 200 tests offered while waiting for its tests to be cleared by the U.S. Food and Drug Administration.

In the study, which was done in July, 14 blood samples were collected from the patients within a 6.5 hour window, including from two separate Theranos retail outlets in Phoenix, Arizona, and test facilities for LabCorp and Quest.

Theranos is still working on publishing its own data, and will have something “in the coming months,” according to spokeswoman Brooke Buchanan. The startup has held research round tables with top experts over the last three months, she said.

Holmes previously said that Theranos had asked the Cleveland Clinic, a prominent research hospital, to run a study comparing its technology to traditional blood draws. As of Jan. 27, the Cleveland Clinic had not started the study.  Eileen Sheil, a spokeswoman for Cleveland Clinic, didn’t immediately respond to a request for an update on the study, and Buchanan declined to comment in detail on any independent studies.