Source: Gilead Sciences

The last time Gilead Sciences (GILD 0.91%) reported quarterly results, the biotech easily trounced analysts' estimates. However, Gilead's stock dropped due to a disappointing outlook for 2015. 

On Thursday of this week, Gilead steps up to bat again with its first quarter results. Will investors be disappointed or delighted? Here are three things to watch that will likely make the difference.

1. Hep C expectations game
There's little doubt that Gilead's two blockbuster hepatitis C drugs, Sovaldi and Harvoni, will once again post massive numbers. In the fourth quarter of 2014, the drugs combined for revenue topping $3.8 billion. The big question, though, is whether or not Sovaldi and Harvoni will perform as well as investors expect.

Michael Yee at investment firm RBC Capital thinks that Gilead's hep C drugs should bring in over $4 billion worldwide in first quarter of 2015. If he's right, that would represent a big win versus consensus expectations of around $3.5 billion. 

Gilead could beat those expectations and prove Yee right if its strategy of expanding access pays off. The company negotiated furiously over the last several months with U.S. payers and European governments to make Sovaldi and Harvoni as widely available as possible. To accomplish this goal, however, Gilead had to sharpen its pencil on pricing. If wider access to the drugs resulted in faster growth than many analysts thought would happen, Gilead will win the hep C expectations game for first quarter.

2. Truvada juggernaut momentum
While hep C gets more attention these days, Gilead's HIV franchise made the biotech the global force that it is today. In the wake of the dizzying success of Sovaldi and Harvoni, it's easy to forget that the company's HIV drugs generated 42% of revenue last year -- thanks primarily to Truvada.

If you just looked at sales by product, you might not fully appreciate the significance of Truvada. After all, it's just one of five blockbuster HIV drugs in Gilead's lineup and isn't even the biggest of the bunch. However, three of them -- Atripla, Stribild, and Complera/Eviplera -- are based on Truvada. 

Strong growth from these HIV drugs could play a key role in Gilead's first quarter success or lack thereof. Especially keep a watch on how Stribild performs. Last quarter, the drug saw nearly 89% year-over-year sales growth. Truvada and its sister drugs might not be basking in the limelight, but don't underestimate the impact that they can make.

3. Other surprises
Other less-watched factors could also make the difference for Gilead in first quarter. Cardiovascular drugs Letairis and Ranexa, for example, kicked in revenue of over $1.1 billion last year and are growing solidly if not spectacularly.

The biggest dark horse that holds some potential to surprise, though, might be Zydelig. In its first full quarter on the market, the blood cancer drug generated sales of only $17 million. Rival drug Imbruvica has clearly outperformed Zydelig thus far. 

Despite its relatively slow start, Zydelig has been projected to achieve peak annual sales between $1 billion and $2 billion. If the first quarter results show the drug picking up steam, Gilead's outlook could appear sunnier than initially expected.

Views
How well Gilead's first quarter is judged will depend mainly on the company's performance against Wall Street's views. Analysts expect earnings of $2.31 per share on revenue of $6.89 billion. Whether or not Gilead hits the mark will be determined largely by the three factors mentioned.

Investors would do well, though, to think less in terms of quarterly performance and more about long term performance. Gilead Sciences continues to be a major force in two important therapeutic areas -- hepatitis C and HIV -- and has the potential to become more of a player in the cardiovascular and oncology arenas. The stock trades at less than 10 times forward earnings. A long-term view will weigh these factors much more heavily than any quarter-to-quarter fluctuations in fortunes.