My date with the phlebotomist was surprisingly convenient. She came to my house in rural Colorado and drew a few tiny tubes of my blood. She then sent those vials to Quest Diagnostics for its Blueprint for Athletes test, which promised to reveal crucial information about how the biomarkers in my blood related to my athletic performance. “No matter your sport or fitness level, the information held in your blood is the missing piece,” their website says. As a runner, cyclist and skier, I assumed that knowing this “missing” information couldn’t hurt, and the company said it would offer personalized advice on what I could do to improve these measures and, presumably, my performance. What could go wrong?

A lot. In the age of the quantified self, any athlete willing to pony up $139 (or more for a more advanced test) can skip her doctor and turn directly to her data. Blueprint for Athletes is only one of numerous direct-to-consumer blood tests targeted at athletes — others include Fuelary, WellnessFX, Athlete Blood Test and Inside Tracker, a company that likens its blood tests to “a selfie from the inside.” The implication is that your blood contains crucial information that you need to know. As Fuelary founder Josh Shadle put it to me, “There could be things going on that you don’t know. … You don’t want to wait until you don’t feel healthy.” It’s this “just in case” anxiety that the tests seem to exploit. If you don’t know your numbers, you might be missing an opportunity to enhance your performance.

Intrigued, I accepted an offer from Richard Schwabacher, executive director of sports and diagnostic solutions at Quest Diagnostics, to find out what the company’s test could tell a middle-aged has-been like me. I received results from 43 tests — including measures of red and white blood cell counts, glucose, triglycerides, hemoglobin, cortisol, cholesterol and vitamin D. Two of my test values, estimated glomerular filtration rate (eGFR) and creatinine, were marked in red — outside of the normal range. Both of these measures have something to do with kidney function, which sounded frightening.

My results came with an appointment with Bunny Foxhoven, a registered dietitian nutritionist and senior clinical educator for Quest. “I wouldn’t worry now,” she said, but she suggested that I get retested in three months to see whether the results were still out of the normal range. The low eGFR number and high creatinine level could signal a kidney problem, she said, or they could mean that I’m eating more protein than my kidneys can easily process.

After asking a bunch of questions about what I eat, she instructed me to cut back on salt and talk to my doctor. “You do want to get on it before it’s at the point where you can’t repair it,” she said of this as-yet-unconfirmed kidney malady. My kidneys could lose as much as 90 percent of their function before I ever get a symptom, she said: “The only way to know is through these random tests, and that’s scary.”

I did talk to my doctor, who advised me not to panic. When you start testing healthy people, she said, it’s not unusual to get some incidental findings — things that look a little off but don’t signify anything worrisome. Michael Joyner, a physician and human physiology researcher at the Mayo Clinic, agreed.

Because my creatinine level was a bit high, Joyner said, it was almost inevitable that my eGFR would also be off, because it is calculated based on creatinine levels. “That’s the easy explanation, but if you showed that data to the average clinician, you could get a big kidney workup out of it,” Joyner said. Had my doctor been less well-versed on the dangers of testing the so-called “worried well,” I could have been sent down a spiral of escalating testing that ultimately cost me money, time and anxiety without making me any healthier.

In fact, when I took a closer look at the written explanation that accompanied my results, I saw that Quest cautioned against reading too much into my creatinine and eGFR levels. According to my results packet, elite athletes tend to have higher than normal creatinine levels, and endurance athletes and team sports athletes may have lower eGFR. The packet said athletes were best off comparing their creatinine and eGFR measures to their own baselines, rather than the normal reference range — the set of values that most of the normal population falls into. In other words, don’t freak out (but keep getting more tests).

My results for the other 41 tests were squarely in the normal range, and I wondered what I was missing. How was any of this going to help me improve my performance and recovery? Was it just a matter of, “Hey, good news — there’s nothing wrong with me”? “OK, good points,” Foxhoven said. She told me to focus on hydration, which Joyner said is “probably the world’s best piece of generic advice to athletes.”

I called Schwabacher to ask him what I’d overlooked. How would these results help me perform better or become healthier? “The question you’re asking is a hard one to answer,” he said. “It really depends on the results.” Had my tests shown some significant deficiency in vitamin D levels or something else that I could correct by changing my diet or taking supplements, he said, my perspective would be a little different: “It’s sort of like good news, bad news. The good news is you’re healthy. The bad news is that sort of takes away some of the value of the test.”

Still, he said, someone whose results were normal might take comfort in knowing that the reason she felt healthy was that she was healthy. He recounted something he heard from an athlete who got a test right before a big race. “Everything was green [normal], but having the peace of mind and knowing my body is where it’s supposed to be gives me confidence going into the race,” he said she told him.

But as my results illustrate, conducting screening tests on healthy people can create as many questions⁠ as it answers. It’s one thing to have symptoms of low iron and then test for iron, Joyner said. It’s another to test someone’s blood without a clear purpose. Measure 40 things in the blood, and chances are, you’ll find something that’s a little fuzzy.

Schwabacher said that my doctor’s advice — not to worry about what was almost surely a spurious result — was just one point of view. Another doctor might have had “an alternate view,” he said, “which is, ‘I’m glad we caught this — there’s something that’s gone undiagnosed.’” Athletes using the test have discovered previously undiagnosed problems, he said. “I think you’d have a different point of view on whether testing healthy people is beneficial” if that had happened to you, he said.

Yet in results from at least one of these testing services, what’s flagged as a problem may be different from what would be considered abnormal on a test you’d get from your doctor. When you get a regular blood test, the results that come back are compared to reference ranges based on the healthy general population, but one private testing service, Fuelary, uses a different reference range.

Fuelary uses a reference range for vitamin D of 50 to 100 nanograms per milliliter, compared to the standard lower cutoff of 20 or 30 ng/ml. When I asked Shadle, the Fuelary founder, to explain the company’s more stringent reference ranges, he told me that they’re set by naturopathic doctors and sent me excerpts from a few papers that do indeed suggest that higher levels might be good for athletes.

But “it isn’t enough to suggest or hint that a blood level is suboptimal,” said Vinay Prasad, a hematologist-oncologist at the Oregon Health and Sciences University. “As a general rule in medicine, you have to show that altering the blood level to what you believe is optimal actually helps.” And after reading the scientific references that Shadle sent me to justify the company’s stricter vitamin D ranges, Prasad said the the evidence is lacking: “The data [Fuelary] cited is mostly expert opinion or observational data on vitamin D levels. Not a single study given shows that altering an athlete’s level to the targets proposed either improves performance or health.”

Another service, Inside Tracker, promises to calculate your optimal blood marker ranges, customized to who you are. The company rates results as “good,” “needs work” or “at risk” based on how they compare with values from people your age, gender and ethnicity. Founder Gil Blander would not divulge where his company’s comparison data comes from except to say that it’s “part of our proprietary intellectual property.” (People who sign up for the tests agree to fine print explaining that their data will become the property of Inside Tracker.)

Encouraging athletes to test themselves with the aim of “improving” their biomarkers, Joyner said, is the sporting equivalent of teaching to the test, rather than training athletes to read their bodies and understand when they’re tired and need rest. What’s important isn’t the number on the result, it’s how an athlete is performing, he said.

These blood tests illustrate a disconnect between innovators in the tech industry and people who know medicine, Prasad said: “They always think more information is better, but those of us who practice medicine know that information is a double-edged sword. Just because you have a number doesn’t mean it’s helpful.” These blood tests, Prasad said, fall prey to the McNamara fallacy — “not everything that counts can be measured, and not everything that can be measured counts.”

Have you used one of these types of blood tests? If so, tell me about your experience!