Why are some investors dropping Gilead Sciences (NASDAQ:GILD), when the company just reported 137% year over year sales growth? As Healthcare contributor Todd Campbell points out, the answer can be seen in which investors are jumping ship and which are staying.

Campbell explains why some short-termers are getting out, and which three factors the long-term Foolish investor should bear in mind in order to see beyond the headlines and make a more informed decision on Gilead Sciences, especially now while the share price is down.

A full transcript follows the video.

Todd Campbell: Gilead Sciences reports a blockbuster quarter, yet shares fall. Here are three things you ought to know, on this health care edition of Industry Focus.

Hello fellow Motley Fools, this is Healthcare Contributor Todd Campbell. Despite us having some technical difficulties at Fool HQ today, I want to take a couple minutes and discuss Gilead Sciences' recently announced fourth quarter and full year earnings results.

In particular, I want to discuss three items that investors may want to consider prior to deciding whether or not to buy or sell Gilead Sciences shares.

However, before I discuss those three points, I want to discuss the elephant in the room, which is why is it that Gilead Sciences shares are tumbling despite the company reporting 137% year over year sales growth in the fourth quarter?

I think the reason for that tumble stems from the fact that there are two separate types of investors. There are short-term investors who tend to jump in and out of stocks, looking for whatever's hot today. Then there are long-term investors who, like most of us at The Motley Fool, like to consider disruptive companies and the impact they can make over a matter of years, not over a matter of days.

In Gilead Sciences' fourth quarter earnings report they issued guidance for 2015. That guidance calls for sales to run at about $26 billion to $27 billion in 2015. At the low end, that would be 4.5% growth over the lofty levels registered in 2014.

Frankly, I don't think that's enough to keep those short-term investors happy, so it's probably not surprising to see shares selling off a little bit as those short-term investors step aside and look for higher-growth opportunities elsewhere.

But for long-term investors, people who are looking for those disruptive companies that can continue to grow sales and profits for years not for days, there are three points that I want to make; three things, if you will, to consider beyond what you're reading in the headlines.

Those three points involve the HIV drug franchise, operating margins, and -- surprise! -- Sovaldi still matters.

First off, let's talk about the HIV franchise. In the past year, most of the attention on Gilead Sciences has focused not on the HIV drugs but on the hepatitis C drugs Sovaldi, which was approved in December 2013, and Harvoni, which was approved in genotype 1 patients in October.

Sure, in the past year those two drugs have combined to generate $10 billion-plus in sales. But the HIV franchise also generates $10 billion in sales per year, and that business is growing by double digits.

In the fourth quarter sales of HIV drugs were up 15.5% to $2.8 billion, and for the full year sales rose 13.2% to $10.1 billion.

Now, Sovaldi did $10 billion in sales. Harvoni, in its first quarter, did $2 billion in sales. Either way, no matter how you look at it, HIV is still going to account for roughly half of Gilead Sciences' revenue in 2015, so investors shouldn't rely solely on the opportunity for growth in hepatitis C. They should also be considering what the potential is in the HIV drugs.

From my perspective, the HIV franchise has plenty of years' worth of running room left to go. One of the reasons that I believe that is because historically adherence rates to HIV medicine were less than they should have been -- less than ideal.

That's because there was a difficulty in getting access to insurance; the high cost of drugs just simply made it so that many HIV patients would have to go off and go on, and go off and go on their medication.

Now, with Medicaid expansion in many states and health care exchanges, those patients are having a much easier time staying on their medication. That's going to mean the scrip volume for HIV drugs is likely to trend higher, not lower.

The other tailwind that's likely to support HIV franchises over the coming years is the fact that HIV patients, because of these advances, are living much longer. Since HIV drugs have to be taken continuously, that means that there's going to be ongoing demand for Gilead Sciences' drugs for years to come.

The second point I wanted to make today is that operating margin at Gilead Sciences is not only strong, but it's growing. In 2013 the operating margin was about 45% and in 2014 the operating margin is about 67%.

Now, there are a number of reasons that operating margin has climbed, but the biggest reason is the fact that volume has grown to such a level that it's being leveraged against fixed costs and that's making the company overall more profitable.

If the company didn't think that those operating margins were going to remain at similar levels, they probably wouldn't have gone out and issued a $0.43 cash quarterly dividend for investors. Frankly, the company has become so profitable, it's kicking out so much cash, that there's a tremendous amount of financial flexibility to not only support the dividend payout, but also to continue to invest in next-generation therapies and the research pipeline.

When investors ask, "What should I be focusing on as far as the financials?" when it comes to Gilead and mature companies about the size of Gilead, I like to look at the operating margin. In this case, that makes me want to be long -- and in fact I am long.

The third point that I wanted to make for investors is that Sovaldi, the drug that was launched in December 2013, isn't going anywhere soon. That drug is likely to remain a major contributor to sales in 2015, and that's because Sovaldi is a pan-genotype drug.

It's not used just to treat genotype 1, which is what Harvoni and AbbVie's competing drug Viekira Pak are approved to treat. It's used to treat genotypes 2 and 3 and 5 and 6, so there's still a tremendous amount of demand for Sovaldi.

To put that demand into perspective, last quarter Sovaldi sales were about $1.7 billion. Now, the company says that about 30% of those sales came from genotype 1. Those genotype 1 sales are probably going to disappear this year, now that Viekira Pak is available.

But even if you lop 30% off of the $1.7 billion, you still end up with a drug that's doing about $1.2 billion in quarterly sales; $1.2 times four gets you $4.8. Harvoni did $2 billion in sales in its first quarter on the market. That's another $8 billion, so you're talking about a 2015 run rate for sales for hepatitis C of about $12-12.5 billion.

Throw on top of that the fact that you've got this HIV drug franchise generating another $10 billion in revenue, you've got operating margins that are accelerating and are very high, respectable, and you've got a recipe that may suggest that Gilead Sciences shares, at least for long-term investors, are offering an attractive entry, especially given the fact that the valuation has gotten even cheaper over the course of the last couple days.

Looking at it earlier this morning on a 2015 basis, the shares were trading at about 9 times estimated earnings.

Those three things are things that I would be considering when it comes to evaluating whether or not I'd want to be long Gilead Sciences following its fourth quarter earnings, and would want to own it in 2015.

I hope you found that helpful. As The Motley Fool, we always want to provide a little bit deeper dive; a little bit deeper, broader understanding of the situations that are affecting companies that we feel offer a lot of interest to investors.

Obviously, if you have any additional questions, feel free to reach out and let us know. Until then, Fool on!