Even after investing in biotech for many years, I still find it difficult to predict how the FDA is going to respond to a drug application. The latest biotech to get rocked was Momenta Pharmaceuticals (NASDAQ:MNTA) and partner Sandoz, a division of Novartis (NYSE:NVS), as the FDA turned down their application for a generic version of Sanofi-Aventis' (NYSE:SNY) Lovenox. Prior to the announcement, I would have put the odds of approval quite high, and I was surprised by the FDA's decision.

As I wrote last month, BioMarin Pharmaceutical (NASDAQ:BMRN) has an FDA decision of its own on the horizon with its drug Kuvan for the treatment of phenylketonuria. During last week's earnings conference call, management announced the FDA action date has been moved from Nov. 25 to Dec. 14 because of staffing issues at the agency. A three-week delay is not a big deal and has no influence on approval odds.

During the call, management also tipped its hand on the likely outcome. They said they were in labeling discussions with the agency. A drug's label is the document containing a thorough description of the drug's mechanism of action, clinical data, dosing and administration, and, importantly, the population of patients for which the drug is approved. That last point dictates how the drug can be legally marketed.

To say they're in labeling discussions is an interesting comment, because I don't think the agency would be hashing out a drug's label if the FDA had problems with the drug's safety or effectiveness and was going to issue an "approvable" letter instead of outright approval. I've heard management teams say in the past they were in labeling discussions, and I don't recall a time where the drug was not approved. I'm sure there have been exceptions, but I don't remember one, and I think this is about as reliable an indicator as you can get as to whether a drug will be approved.

I now feel even stronger about my conviction that Kuvan will be approved next month. This is a very important drug for BioMarin. It should allow the robust double-digit sales growth to continue for years and push the company into profitability.

Fool analyst Charly Travers does not own shares of any company mentioned in this article. BioMarin and Momenta are Rule Breakers recommendations. The Fool has a disclosure policy.