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Once difficult and expensive even for the most technologically advanced labs, genetic testing is fast becoming a cheap and easy consumer product. With a little spit and 200 dollars, you can find out your risk for everything from cystic fibrosis to lactose intolerance.

But it’s important to remember that not all genetic tests are created equal. And even the best clinical genetic test, carried out in a medical lab under a doctor’s supervision, isn’t perfect—genes are important, but they don’t seal your fate.

Who should get a genetic test?

Genetic tests are diagnostic, so anyone who is curious about their health can get one done. But they’re more informative if you think you might be at risk for a genetic disorder.

Heavy-duty genetic tests have been used as a clinical tool for almost half a century—long before 23andMe and began offering direct-to-consumer tests. Let’s say that many women in your family have had breast cancer. You can get a genetic test to see if you may have inherited an abnormal version of the BRCA gene, known to increase your risk for breast cancer.

Heidi Rehm, associate professor of pathology at Harvard Medical School, is the director of the Laboratory for Molecular Medicine, where patients get tested for diseases that can be traced to specific genetic roots. She says it is most common for people to get tested when they either suspect or know that they have a genetic disease; it may have affected multiple people in their family or they could show symptoms of something widely known to be genetic, like sickle cell anemia. For these people, genetic tests can provide a much-needed explanation for an illness and help doctors determine the best course of treatment. Babies are often tested for genetic diseases, either while they are still fetuses or shortly after birth.

Others get genetic tests if they and their partner both have family histories of an inherited disease—even if they don’t have the disease themselves. For example, cystic fibrosis is linked to one particular gene, but you have to inherit the abnormal version of the gene from both your parents to get the disease. If you only inherit one copy, you may never know—you won’t display any of the symptoms. But if you and your partner both carry one copy of the faulty gene, your child could still inherit two copies. Genetic tests can forewarn you of that possibility.

But Rehm says there has been a recent trend of healthy people getting tested to predict whether they’ll get certain diseases. “I do think there are settings where predictive genetic testing is incredibly important and useful,” Rehm says; for example, knowing that you’re at risk for breast cancer gives you the opportunity for early intervention (remember when Angelina Jolie got a double mastectomy upon finding out she had a mutated BRCA gene?)

But Rehm also points out that genetic tests may not be as straightforward as they seem. For example, some genes are thought to increase risk of getting a certain disease, but it might only happen if you have specific family history, or you might be able to reduce your risk with lifestyle changes. So remember that a genetic test isn’t the final verdict—there are other factors at play too.

Can a genetic test predict my medical future?

Not entirely—its scope is limited. For starters, not all diseases are caused by genes. Plenty of conditions stem from environmental and lifestyle factors; they may interact with your genes, but the external factors are the real trigger.

But even if a disease is caused solely by faulty instructions written in your genes, you won’t necessarily be able to test for it. That’s because genetic tests are mainly used for diseases that are “penetrant,” a term that scientists use to describe a strong connection between having a certain gene (or multiple genes) and getting a disease.

Genetic tests are surprisingly simple on the surface. All that’s required of you is a small sample of cells, like a blood sample or saliva (which doesn’t have DNA itself, but picks up cheek cells during its journey out of your mouth). It get sent to a lab where sequencing machines match up small pieces of synthetic DNA with your DNA to figure out the overall sequence.

Once they have your sequence, geneticists can compare it with “normal” or disease-causing sequences. In the end, they might give you a “yes” or “no” answer, or sometimes you’ll get a probability—a measure of how much your genes increase your risk of developing the disease. Then, it’s up to your doctor to figure out what these genes (in combination with your lifestyle, family history and other risk factors) mean for your health.

With penetrant diseases, there’s a “very, very high” ability to explain the disease, Rehm says. For example, the breast cancer-related gene BRCA1 can give you a 60 percent chance of getting breast cancer (in Jolie’s case, with her family history, the risk was 87 percent.)

This makes genetic tests better at detecting so-called “rare diseases,” says Steven Schrodi, associate research scientist at the Marshfield Clinic Research Institute’s Center for Human Genetics, but they’re less useful when it comes to more common diseases, like heart disease or diabetes. Genetics can increase your likelihood of getting these disease, but scientists still don’t know quite how much. Part of the problem is that there may be dozens or hundreds of genes responsible for these diseases, Schrodi says.

“We have an incomplete understanding of why people get diseases,” Schrodi says. “A large part of it hinges on how we define diseases. Perhaps physicians have inadvertently combined multiple diseases together into a single entity.”

Consumer genetic tests—the ones where you send in samples from home—sometimes claim to test for these more complex traits, but be careful: Their results might not be very medically relevant, Rehm says. If they tell you that your genes make you twice as likely to develop diabetes, for example, that’s a marginal increase that doesn’t significantly affect your risk, especially when you take into account lifestyle factors.

Can genetic tests predict how long I will live?

Genes do seem to play a role in determining lifespan. After all, some family reunions stretch from great-great-grandparents all the way down to infants. Scientists have studied centenarians—people who lived to be 100 years old—and found that people with certain versions of genes involved in repairing DNA tend to live longer.

This makes sense because aging leaves its mark on your DNA. Environmental factors can damage DNA, and even the routine chore of replicating cells can introduce errors as the three billion units of your DNA are copied over and over. Long-lived individuals have different sequences that seem to make their cells better at keeping DNA in mint condition.

But figuring out your expiration date is more complex than just testing for a few genes, says Jan Vijg, professor of genetics at Albert Einstein College of Medicine. In theory, you could design a test that looks at specific genes that might measure your risk for developing Alzheimer’s Disease or other age-related diseases, or your risk for aging quickly. “To some extent, yes: Biomarkers will tell you something about your chances of living a long life,” Vijg says. “Still, that will only work if you live a careful life.” And that means no accidents, infections, or cancers.

Aging also affects the exposed ends of your DNA, called “telomeres.” DNA is stored as chromosomes, those X-like structures that you may have seen in biology textbooks. The most vulnerable parts of the chromosome are the chromosome’s tips, which get shorter as you age because they aren’t properly replicated. But while telomere length might let you compare your DNA now with your DNA from a decade ago, you can’t compare your own telomeres with other people’s telomeres. There’s a lot of variation between individuals, Vijg says. Some of us are just old souls (on the genomic level, that is.)

The methylation test, which looks at how the presence of small chemical groups attached to your DNA changes as you age, might be a better bet. A study at UCLA showed that changes were slower in longer-lived people. But Vijg is hesitant: “I would not put my hopes on that as a marker to predict when exactly you’re going to die.”

For now, just enjoy your life, because you can’t predict death. And if you decide to unlock the secrets of your DNA with an at-home test, don’t take those results for more than their worth.