The good news keeps coming in for Biogen (BIIB 0.63%). On Tuesday, the drugmaker reported its third-quarter results, and for the first time in a while, investors found something in one of its earnings updates to cheer about.

Biogen beat Wall Street's expectations on both the top and bottom lines. The company also increased its 2022 revenue and earnings outlook. So is Biogen stock a buy now?

Largely irrelevant

Let's first examine the good news in Biogen's Q3 results. Yes, revenue of nearly $2.51 billion was better than the consensus estimate of $2.47 billion. The company's adjusted earnings of $4.77 per share also blew the average estimate of $4.14 per share out of the water.

However, there's more to the story. Biogen's revenue fell by 10% year over year. Its profits increased only because the company significantly reduced its spending.

Even with the increased full-year guidance, Biogen will continue to move in the wrong direction financially. The company raised its 2022 revenue outlook to a range of $10 billion to $10.15 billion from its previous forecast range of $9.9 billion to $10.1 billion. Its adjusted earnings guidance is now $16.50 per share to $17.15 per share, up from the prior range of $15.25 per share to $16.75 per share. However, the upper ends of both updated ranges still imply year-over-year declines.

In a real sense, Biogen's Q3 results were largely irrelevant. But it wasn't because of the numbers themselves. 

What really matters for Biogen

The most important discussion in the biotech's latest update centered on lecanemab. On Sept. 27, Biogen and its partner, Japanese drugmaker Eisai (ESALY -0.68%), announced results from a late-stage clinical study evaluating the drug as a treatment for Alzheimer's disease. You only have to look at the huge pop Biogen stock took that week to get a sense of just how positive those results were.

Biogen CEO Michel Vounatsos' first comments in the Q3 earnings call focused on lecanemab. That's not surprising in the least. The Alzheimer's disease drug truly could be a game-changer for the company.

You could even argue that lecanemab might be a lifesaver for Biogen as well. As the company's Q3 results showed, overall sales for its currently approved drugs continue to decline. Alzheimer's disease drug Aduhelm was a bust after the Centers for Medicare and Medicaid Services decided that Medicare would not cover the cost of the drug except for use in approved clinical studies. Now, though, Biogen and Eisai are bringing lecanemab to the table -- a drug that appears to slow the progression of Alzheimer's disease.

The two companies won't have to wait long to find out if they'll be able to market lecanemab in the U.S. The Food and Drug Administration set a PDUFA date of Jan. 6 to make its decision on accelerated approval of lecanemab. Eisai also plans to file for approvals in Japan and Europe and for full "traditional" approval in the U.S. by the end of Q1 2023.

Buy Biogen stock?

Let's return to the initial question: Is Biogen stock a buy now? It's clear the company's improved outlook for 2022 really doesn't matter much. But lecanemab holds the potential to change the dynamics for Biogen.

Analysts estimate that the Alzheimer's disease drug could generate peak annual sales of between $6 billion and $10 billion. Biogen stands to receive 50% of all profits made by lecanemab globally. 

The company's pipeline also features several promising programs that are either awaiting approval or in late-stage testing. Zuranolone, an experimental drug being developed by Biogen and Sage Therapeutics for treating depression, stands out as an especially noteworthy candidate.

My view is that Biogen's current market cap of around $40 billion doesn't fully reflect the potential for lecanemab and other products in its lineup and pipeline. So I do think that the biotech stock is now a buy -- but that opinion has nothing to do with its increased full-year guidance.