Rapid manufacturing expert Proto Labs (PRLB -0.13%) reported another record quarter of earnings results, driven by an impressive 21% increase in customers and an equally impressive 9% increase in spending per customer. The stock initially sold off because the company slightly missed analyst expectations with its first-quarter outlook. As a result, this headline "disappointment" may present a great buying opportunity for Foolish investors that love buying excellent businesses for even better prices.

Breaking down the "disappointment"
Next quarter, Proto Labs believes it will generate between $45 million and $48 million in revenue and earn between $0.36 and $0.40 a share on an adjusted basis. Although these numbers suggest that Proto Labs will grow its revenue by at least 20% year over year, the midpoint of its guidance fell just shy of expectations, calling for $46.5 million in revenue and $0.40 a share in earnings. Foolish investors should try to divorce themselves from the fickle game of Wall Street expectations and instead focus on how the underlying business is performing.

In the fourth quarter, revenue soared 31% year over year to a record $44 million, which translated to a record net income of $9.5 million, or $0.36 per share. Gross margin edged up slightly to 62.7%, operating margin increased 1% to 31.5%, and R&D spending increased to $3.5 million or 7.9% of revenue. Together, these metrics indicate that Proto Labs operated more efficiently than it did the year before.

Proto Labs ended the year with over $143 million in cash and practically zero debt, giving it plenty of firepower to grow its operations and make the transition to a new CEO.

Healthy vitals
The most important metric for Proto Labs investors to watch is how many customers Proto Labs serves in a given quarter and how much spending each customer generates. Tracked over time, it can tell you how Proto Labs' business is performing and whether or not the long-term-investment thesis remains intact. In an ideal world, you'll want to see the number of customers served increase and spending per customer to rise. This will give you the confidence that Proto Labs' rapid manufacturing services remain in strong demand, and that it's creating a base of satisfied and loyal customers to help drive longer-term results. It would be unlikely for overall spending per customer to rise and yet have a dissatisfied customer base.

In other words, the 21% increase in customers served and the 9% increase in spending per customer during the quarter is a very encouraging sign for the future.

Taking advantage of the opportunity
Investors thinking about taking advantage of this buying opportunity should also consider the price of the stock, which isn't cheap by traditional metrics, whether or not you believe in Proto Labs' newly appointed CEO, Victoria Holt, who happens to be an expert in driving new levels of growth, and the company's opportunity to grow in the world of rapid and small batch manufacturing.

When you put it all together, you get an obsessive customer-first organization that leverages technology and automation, which has allowed it to become the world's fastest rapid manufacturing service. With a relatively small market cap of $2 billion, considering its disruptive manufacturing potential, I think today's earnings report reinforces why Proto Labs deserves a chance to be part of your portfolio.