What: Shares of NetScout Systems (NASDAQ:NTCT), a provider of application and network management solutions, tumbled on Thursday after the company reported its fiscal third-quarter results. At 3 PM EST Thursday, the stock was down about 20%.

So what: NetScout reported quarterly GAAP revenue of $307.7 million and non-GAAP revenue of $333.4 million, up 150% and 171% year over year, respectively. The third-quarter results include a full quarter's contribution from Danaher's Communications Business, which the company acquired in late 2014 for $2.6 billion in stock. NetScout's GAAP revenue missed analyst estimates by about $5 million.

NetScout posted non-GAAP earnings of $0.58 per share, down $0.01 compared to the prior-year period, but $0.18 higher than the average analyst estimate. On a GAAP basis, the company posted a loss of $0.25 per share, compared to a gain of $0.42 per share during the same quarter last year. The discrepancy was mostly due to the amortization of acquired intangible assets, along with a handful of other items.

Now what: While NetScout's results weren't bad, the company cut its guidance for fiscal 2016. GAAP revenue, which is now expected to be between $958 million and $968 million, down from a previous range of $997 million to $1.047 billion. The company also expects a bigger GAAP net loss, guiding for EPS in the range of ($0.21) to ($0.29), down from a previous range of ($0.10) to ($0.25).

The guidance cut seems to be the main driver behind the slumping stock price. CEO Anil Singhal pointed to an uncertain macroeconomic environment, but remained optimistic that results would improve in fiscal 2017:

We've seen concern within our customer base, especially in the service provider market, regarding their own plans for calendar year 2016 due largely to the uncertain macroeconomic environment. As a result, we've taken a more cautious and conservative review of our customers' projects and anticipated near-term spending plans. At this time, we believe it is prudent to revise our fiscal year 2016 guidance with a focus on preserving our earnings performance. We remain confident that our value proposition, breadth and depth of innovative solutions, excellent financial foundation and long-standing customer relationships will continue to serve us well as we move forward. We are in the early stages of an exciting transition from legacy products to integrated solutions that we believe can leverage our differentiated software technology, drive further traction with customers worldwide and help us further accelerate margin improvement moving into fiscal year 2017.

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