What happened Shares of Fulgent Genetics (NASDAQ: FLGT) soared nearly 31% at one point Friday morning. However, the stock was up only 5% as of 11 a.m. EST. That huge, albeit temporary, gain stemmed from Fulgent's fourth-quarter update after the market close on Thursday. The genetic testing company reported revenue of $295 million with adjusted earnings of $167.5 million, or $6.20 per share. Both top- and bottom-line results represented huge improvements from the prior-year period and trounced Wall Street consensus estimates. Fulgent also provided 2021 guidance that projects 90% revenue growth this year. Image source: Getty Images. So what Fulgent delivered jaw-dropping growth. It blew past analyst expectations. The company's guidance was terrific. It's understandable why shares would jump as they did earlier today. But why did the healthcare stock give up most of its gains? It's probably because investors aren't convinced the growth is sustainable. Most of Fulgent's tremendous revenue increase came from its COVID-19 testing business. Based on the company's guidance, this business should enjoy strong demand in 2021. However, there's a big question as to how long it will last. As COVID-19 vaccines become more widely available, the pandemic could come to an end. While investors are excited about Fulgent's Q4 results, many of them probably also worry that the company's growth could taper off dramatically in the not-too-distant future. Now what One key thing to watch with Fulgent going forward is the growth in its non-COVID-19 business. The company said that its non-COVID-19 genetic testing revenue jumped 43% year over year in Q4. It's also possible that concerns about a steep decline in COVID-19 testing could be premature. The emergence of new coronavirus variants could mean that Fulgent Genetics' COVID-19 revenue will remain strong for a long time to come. 10 stocks we like better than Fulgent Genetics, Inc.When investing geniuses David and Tom Gardner have a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* David and Tom just revealed what they believe are the ten best stocks for investors to buy right now... and Fulgent Genetics, Inc. wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of February 24, 2021 Keith Speights has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Fulgent Genetics, Inc. The Motley Fool has a disclosure policy.Source