The Lab Says It’s Cancer. But Sometimes the Lab Is Wrong.

Ultimately he had his prostate removed. “I said, ‘Mistakes happen,’” Mr. Karman said.

They may be happening more often than doctors realize. There is no comprehensive data on how often pathology labs mix up cancer biopsy samples, but a few preliminary studies suggest that it may happen to thousands of patients each year.

Fortunately, there is now a high-tech solution: a way to fingerprint and track each sample with the donor’s own DNA.

But it costs the patient about $300 per sample, and labs have been slow to adopt it, saying that the errors are rare and the test too expensive, and that they have plenty of checks in place already to avoid mix-ups.

Dr. John Pfeifer, vice chairman for clinical affairs in the pathology and immunology department at Washington University School of Medicine in St. Louis, who has studied the problem, is not quite so sanguine.

“All the process improvement in the world does not get rid of human errors,” he said. “Millions get biopsies every year. Is society going to say, ‘Yeah, mistakes happen but we’re not going to look for them?’”

The fingerprinting method, offered by Strand Diagnostics, is simple: A doctor gets a DNA sample by swabbing inside a patient’s mouth. It is sent directly to Strand with a bar code identifying the patient.

That bar code is also used to label the patient’s biopsy. If it shows cancer, the pathologist sends the biopsy cells to Strand. The lab matches the DNA from the swab to that of the biopsy cells.

If these DNA fingerprints did not match, that signaled a lab mix-up. That was how pathologists discovered that samples from Mr. Erickson and Mr. Karman had been switched.

Despite the best efforts of pathologists to avoid these mix-ups, hints of trouble have been turning up for years.

In 2011, researchers conducting a large clinical trial reported that two men who were found to have prostate cancer — and who had…

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