Proto Labs (NYSE:PRLB) is scheduled to report its third-quarter earnings on October 22 before the market opens. Wall Street analysts expect the quick-turn manufacturer to grow sales by nearly 23% year over year to $67.1 million and earn an adjusted $0.53 per share. If Proto Labs reports sales greater than $64 million, it'll mark the 13th consecutive quarter of record revenue for the company.

Beyond the headline results, investors should also familiarize themselves with the underlying drivers of Proto Labs' business to help determine if the company remains on track toward its long-term goals. Overall, there are three key areas for investors to watch when Proto Labs reports earnings.

1. Product developers served
The number of product developers that Proto Labs serves in a given quarter is one the most important figures it reports because it highlights how the company's rapid manufacturing services are faring in the marketplace. Ideally, investors want to see this figure rising on an annual basis, which would help validate that Proto Labs' manufacturing services are increasing in popularity within the product developer community.

During the second quarter, Proto Labs served a record 11,822 product developers, representing an increase of 28% year over year and a 7% increase sequentially. Although a large driver of this annual growth can be attributed to incorporating the Fineline 3D printing services acquisition into this year's results, the company has an established history of growing the number of product developers it serves dating back to the first quarter of 2012.

Proto Labs Customers Served

Data source: Proto Labs.

In other words, investors will want to see that Proto Labs extended its track record of growing its presence among product developers in the third quarter.

2. Update on Proto Labs' newest acquisition
Near the close of the third quarter, Proto Labs announced the partial acquisition of Alphaform AG, a Germany-based 3D printing and manufacturing service bureau. The purchase brings Alphaform's injection molding and 3D printing segments in Germany, Finland, and the United Kingdom, under Proto Labs' European operations. It also gives Proto Labs a larger presence in Germany, which happens to be the company's largest European market, and expands its European 3D printing service offerings to include selective laser sintering, metal 3D printing, and additional stereolithography capacity.

Ultimately, this acquisition appears to be about Proto Labs continuing to drive growth out of Europe, but investors should listen in on the conference call to hear management's full view on why it purchased Alphaform.

3. Gross margin
As Proto Labs has grown the number of product developers it serves, it has come mildly at the expense of its gross margin, largely because 3D printing services tend to carry a lower margin than manufacturing services, and 3D printing services have grown to represent a greater percentage of the company's total revenue.

PRLB Gross Profit Margin (Quarterly) Chart

PRLB Gross Profit Margin (Quarterly) data by YCharts.

Eventually, investors may grow weary of Proto Labs' gross margin pressures, because it becomes increasingly difficult to drive earnings to the bottom line. After all, for Proto Labs to maintain its earnings, it has to offset any decline in gross margin with increases in sales or decreases operating expenses.

Consequently, investors will want to monitor how Proto Labs' gross margin did in the third quarter.

All eyes on Thursday
When Proto Labs reports earnings on Thursday, focus on how the underlying business is performing, rather than how investors react to the news. Key in to the number of product developers served, an update on its latest acquisition, and how margins fared during the quarter. These areas will likely prove to be great places to start assessing whether Proto Labs' business and growth strategy remain on track.

Steve Heller owns shares of Proto Labs. The Motley Fool owns shares of and recommends Proto Labs. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.