I've taken DNA tests from Ancestry and 23andMe. Here's the one critical thing you need to know before you buy a kit.

• About four years ago, I sent in my samples of spit to 23andMe and Ancestry to find out what my DNA could tell me about my family history and health. I've also taken the now-shuttered National Geographic test.
• At the time, I thought that the initial reports would be the only time I looked at my results. Instead, I've found myself turning back to them frequently and finding new updates.
• In one case, my ancestry results in one update looked completely different from the next one, challenging my understanding of my family history.
• I'm often asked which test I'd recommend. For years, my answer boiled down to one question: What do you want to get out of the test?
• As the distinctions between the tests become less clear, the one consideration I hope people will keep in mind if they take a test is that you shouldn't be surprised if your results change, especially with genealogy.
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I've sent my spit off for more genetics tests than anyone else I know.

It's an occupational hazard. As a healthcare reporter for Business Insider, I've kept a close eye on the consumer genetics test-makers that take my sample of spit and analyze the DNA within it to find out a host of things about my ancestry and health. For years, consumer genetics has been growing in popularity, though there are now some signs that the industry is slowing down.

Genetic-testing companies have proprietary sets of data and various ways of analyzing information, so each one I tried offered a distinct approach. One provided details about my great-grand relatives, while others listed how much Neanderthal DNA I have.

Over the years, I've gone back to my results time and again to find in some cases new reports. One test I've taken, National Geographic's, has since started shutting down.

But one thing I hadn't fully expected when I sent off my first sample of spit: The results for my genealogy keep changing.

Every so often, someone asks me which test I recommend. My answer used to boil down to one question: What do you want to get out of the test?

But as the information the tests provide grows more similar -Ancestry in October launched a health test in addition to its family history reports - my advice has increasingly turned to setting expectations for what people might find out in their reports. And beyond that, the fact that their results could change.

Here's a breakdown of two consumer genetics tests I've taken: 23andMe and Ancestry.

23andMe has given me a comprehensive picture of my health and ancestry that keeps growing

23andMe currently offers two versions of its test: The $199 version comes with health, traits and ancestry components, whereas the $99 version now comes with ancestry and traits, which tell you information about your predisposition to certain traits, like what ice cream flavor you're likely to prefer.

The company now also offers a VIP service that includes two kits, priority lab processing, and one-on-one ancestry walkthroughs for $499.

To analyze your DNA, 23andMe uses a technique called genotyping. Humans have 3 billion base pairs of DNA in our genome - that's a lot of information to sift through - so genotyping technology looks for specific parts of DNA and pieces them together.

The health reports can tell you information about your physical traits (like if you're likely to have dimples or curly hair), wellness (such as how well you metabolize caffeine or if you're a sprinter), and carrier status for certain genetic mutations.

The Food and Drug Administration now allows 23andMe to provide reports on a person's genetic risk for certain diseases, including Alzheimer's, Parkinson's, and certain mutations associated with an increased risk for breast cancer. In total, the test currently has more than 80 reports, and more get added all the time. I often get emails telling me that a new test is ready for me - recently I got one that looks at my genetic risk for celiac disease.

With 23andMe's ancestry reports, users have access to information about their ancestry composition (which geographic regions your genes align with), haplogroups (genetic populations that share a common ancestor), and Neanderthal ancestry. Customers also get access to something called a DNA Relatives tool, which 23andMe users can opt into as a way to connect with other users in the system who might be relatives.

In 2018, 23andMe updated its ancestry reports to provide more specific regional information. My report used to specify just Scandinavian ancestry, but then pinpointed Norway as a country where my ancestors lived within the past 200 years.

But just a year later, my results had changed again. Instead of being roughly 63% Scandinavian, I was up to 66% Scandinavian, a small shift but a reminder at how the datasets can change and influence my report.

Interestingly, the report didn't break down what percent Swedish and what percent Norwegian I was. Instead, it gave me a look at the regions within both countries I like have ancestors from.

23andMe also maps out how many generations ago your ancestors may have lived in a particular region. For example, I may have had a Finnish ancestor sometime in the mid-to-early 1800s or late 1700s, while my French and German ancestors date even earlier.

Verdict: If you're looking at this test as a science experiment, using it as a way to get involved in research, or viewing it as a chance to learn about your genetic health risks, then this is a fit for you. But if you opt for the full test, there are some considerations that patient groups and genetic counselors would like users to take into account.

If you just want to know your ancestry percentages, how many Neanderthal variants you have, and some trait reports, the $99 version is a good bet. Just be prepared to keep checking back in to see how your results might have changed.

Ancestry's DNA tests gave me starkly different ancestry results as I wait to find out what the testing giant can tell me about my health

Ancestry in October made a massive change to its business by getting into the health reports part of the consumer genetics business. It does still offer a $99 ancestry-only version of the test.

Compared to 23andMe, over the years, I didn't find a lot of variation in my Ancestry results until February 2019. When I went back in for another look, there were some pretty big changes.

When I first got my results, I was told I was 90% Scandinavian, not breaking out Norway from Sweden. Then, according an update in September 2018, I was a whopping 71% Norwegian and 17% Swedish. Finding out I was just 17% Swedish was a major shock. By my calculations, I should be about 37.5%.

I brought up the massive discrepancy often in conversations with my family for the next few months, though, at the end of the day it didn't really change any of our traditions.

But in anticipation of the new health tests, I went back to my Ancestry results in October. What I found was another big switch. This time, I found results that were much closer to what my family had told me: 54% Norwegian and 36% Swedish.

The big swings in my results have been a topic I bring up any time the conversation around genetic tests comes up (which, for me, is pretty frequent). I hadn't expected the changes to bug me as much as they have. It has me reconsidering how much stock I put into the genetic reports I get back compared to the recollection of ancestry from my family members.

Ancestry also has a DNA story element that maps out your ancestors' migration patterns. My ancestors started moving to the Midwest in the US around 1825-1850.

Ancestry plans to offer two health products, and I'm still waiting to check them out:

• AncestryHealth Core, which will provide health reports about carrier status for rare conditions parents could pass on to kids, like cystic fibrosis, inherited cancers, and heart disease, and wellness information about nutrition and metabolism. AncestryHealth Core will cost $149, or $49 if users have already taken an Ancestry test, and users can sign up for the test now.
• AncestryHealth Plus, which will roll out in 2020, will offer more reports and the plan is to add additional info over time as the science advances. The test will be run on next-generation sequencing technology rather than the genotyping technology Core and the standard AncestryDNA test are run on. Unlike genotyping, which looks for specific parts of DNA and pieces them together, next-generation sequencing looks at only the protein-encoding parts of your genome, called the exome. The next-generation sequencing analyzes roughly 2% of those 3 billion base pairs. To perform the next-generation sequencing test, Ancestry's partnering with lab-testing firm Quest Diagnostics. The test will cost $199, which includes six months of quarterly updates and additional educational resources. After that, it's $49 every six months for the quarterly updates, or $100 a year.

Users who have already submitted their spit to Ancestry before don't have to submit a new samples. The products will take about 6-8 weeks to report after ordering, even for those who have already taken the Ancestry test.

Verdict: The jury's out until my health report comes back in. In the meantime, if the idea of tracing your family tree through the generations and connecting that information with historic documents gets you excited, this is the test for you.

Privacy considerations

Beyond the consideration of understanding that your ancestry results might change, there's one other big thing to consider before taking a test: privacy.

The tests do, after all, deal with information that's fundamental and unique to every individual.

In a blog post published in December 2017, the Federal Trade Commission recommended reading the fine print.

"If you're thinking about buying an at-home DNA test kit, you owe it to yourself - and to family members who could be affected - to investigate the options thoroughly," it said.

James Hazel, a post-doctoral research fellow at Vanderbilt University's Center for Biomedical Ethics and Society, has been looking into the privacy policies of consumer genetics tests. He said the FTC's suggestion is very important.

"We are good at clicking 'agree' and not reading the terms of service," he told Business Insider in December 2017.

Questions to keep in mind when reading through the terms of service include:

• Who owns your DNA?
• Who gets to see your de-identified (not attached to your name) information?
• How is the data that's tied to your identifiable information used?
• Can you opt out of giving research partners your genetic data?
• Can you wipe your information after taking a test?