An analysis of clinical laboratory tests in 60 healthy adults found “systematic biases” in total cholesterol, low-density lipoprotein (LDL) cholesterol and high-density lipoprotein (HDL) cholesterol values reported by Theranos, a privately-held testing laboratory.

Tests for total cholesterol at Theranos were on average 9.3 percent lower than those performed at Quest Diagnostics and LabCorp, the two largest testing laboratories in the U.S.

Lead researcher Brian A. Kidd, PhD, of the Icahn School of Medicine at Mount Sinai in New York, and colleagues published their results online in The Journal of Clinical Investigation on March 28.

The Wall Street Journal reported that Theranos sent a letter to The Journal of Clinical Investigation last week calling the study “flawed and inaccurate.” The newspaper also said that Theranos criticized alleged conflicts of interest with two of the study’s authors, which the researchers denied.

The Icahn Institute for Genomics and Multiscale Biology and the Harris Center for Precision Wellness at the Icahn School of Medicine at Mount Sinai provided funding for the study.

“While most of the variability we found was within clinically accepted ranges, there were several cases where inaccurate results would have led to incorrect medical decisions,” study researcher Joel Dudley, PhD, of the Icahn School of Medicine at Mount Sinai, said in a news release. “We hope this study will inspire the biomedical community to take a critical look at all testing variables to ensure that lab results are as robust and reproducible as possible.”

Theranos uses capillary tubes to collect blood and claims it has several advantages over traditional models, according to the researchers, including lower collection volumes, convenience and reduced costs.

Theranos collects blood through a finger prick, whereas Quest Diagnostics and LabCorp collects blood through venipuncture.

In early March, Theranos responded to a survey from the Centers for Medicare & Medicaid Services that found issues at Theranos’s lab in Newark, California. Although the survey has not been publicly released, the Wall Street Journal reported that Theranos ran a hematology test on 81 patients despite erratic quality-control results.

For this study, the researchers collected samples from 60 healthy adults from July 27 to July 31, 2015. They collected 14 samples per person and obtained 22 clinical lab measurements per person for a possible total of 18,480 measurements.

In all, they assessed 2,640 possible measurements from Theranos, 7,920 possible measurements from LabCorp and 7,920 possible measurements from Quest and found missing data rates of 2.2 percent, 0.2 percent and 0 percent, respectively. Theranos returned missing data for four adults, whereas LabCorp returned data from one adult.

Further, measurements outside the normal ranges were found in 7.5 percent of Quest’s samples, 8.3 percent of LabCorp’s samples and 12.2 percent of Theranos’s samples.

The researchers said that test results from Theranos were considered abnormal 1.6 times as often as results from Quest Diagnostics or LabCorp. They added that the samples had some measures such as red blood cell counts and triglycerides that agreed between the companies, but white blood cell counts, cholesterol and HDL cholesterol differed significantly.

They also cited a few study limitations, including that they did not have technical replicates in the Theranos data. They also mentioned that samples were shipped from the retail locations to a central facility in California.

In addition, they did not have technical details on the analytical instruments used for the laboratory tests. Thus, they mentioned the results could be due to differences in collection methods, processing, instrumentation or a combination of these factors.

“These testing disparities occurred despite rigorous laboratory certification and proficiency standards designed to ensure consistency,” Eric Schadt, PhD, of the Icahn School of Medicine at Mount Sinai, said in a news release. “Our results suggest the need for greater transparency in lab technologies and procedures, as well as a much more thorough investigation of biological mechanisms that may contribute to more dynamic levels than we currently understand.”