I am a neurosurgeon who opens skulls for a living, yet two tubes of blood left me unexpectedly nervous. Previous blood tests had felt routine, even when they revealed cholesterol or blood sugar levels that improved after dietary changes.

This draw felt different because it could reveal nothing or reshape the next year of my life. The test was Galleri, made by Grail, and it belongs to a class known as multicancer early detection tests.

Sometimes called liquid biopsies, these tests search the bloodstream for DNA fragments shed by tumors. Test makers say a single blood draw can detect signs associated with more than 50 cancers, potentially exposing disease before symptoms appear.

My friend Katrina Firlik, also a neurosurgeon, took Galleri on a whim despite feeling healthy and having no symptoms. Her result was positive, and a subsequent PET scan revealed what she described as a bright lump, “like a light bulb,” in her chest.

The diagnosis was Hodgkin lymphoma. Her thoracic surgeon told her that he rarely encountered tumors that small, making her unlucky to have cancer but fortunate that it had been discovered so early.

Firlik underwent treatment before the disease made her sick. “I didn’t have to survive a cancer crisis. I got to skip it entirely,” she told me.

Her experience made early detection feel irresistible. Screening and prevention efforts between 1975 and 2020 are estimated to have prevented roughly 4.75 million cancer deaths in the United States, representing a major public health achievement.

Yet enthusiasm for finding cancer early is not the same as confidence in every test. Galleri can have strong positive predictive value, meaning a flagged result carries legitimately high odds of representing a real cancer.

The more difficult question concerns negative results. A test must reliably communicate what it means when it finds nothing, particularly if patients interpret that result as permission to skip established cancer screenings.

Dr. Vershalee Shukla runs a cancer screening program for Arizona firefighters, who develop cancer at an elevated rate because of chronic workplace exposure to toxins and carcinogens. Over five years, Shukla has conducted close to 10,000 liquid biopsies and caught fewer than a dozen cases.

During one week, her team collected blood from 500 firefighters in a single day, and every result was negative. Many participants then avoided their regular screenings because the Galleri findings gave them a false sense of security.

Shukla and her team spent nearly two years persuading those firefighters to resume the screenings they had skipped. When they finally returned, 15 were found to have cancers.

“It’s just not picking up cancers,” Shukla said. Grail also emphasizes that Galleri should supplement rather than replace routine screening tests.

“All of our physician and patient materials are explicit that Galleri is not a replacement for routine screening and that it should be used in addition to routine screening tests,” the company said in an email.

Positive results can create their own difficulties because not every cancer discovered early requires immediate treatment, or any treatment at all. Some tumors grow so slowly that they will never cause trouble during a patient’s lifetime.

Finding such a tumor can trigger what doctors call a cascade of care, including more scans, biopsies, waiting rooms and anxiety. Despite the technology’s promise, lengthy trials have not yet proved the most fundamental point: whether these tests save lives.

I took Galleri despite those uncertainties and found the wait difficult, especially while thinking about my wife, children and how I might explain a positive result. The process did not clear me or eliminate the need for scheduled screenings, including a colonoscopy around my birthday.

The experience cost about $1,000 and renewed my appreciation for colonoscopies and mammograms, which have evidence that they save lives. Three multicancer tests on the United States consumer market are Galleri, Abbott’s Cancerguard and Caris Life Sciences’ Caris Detect, with out of pocket prices ranging from $700 to $3,500.

Advisers to the US Food and Drug Administration voted Wednesday that Galleri was safe and effective for certain patients and that its benefits outweighed its risks. An FDA approval could encourage insurance coverage, but the central questions about broad testing and mortality remain unresolved.

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