Is it the flu?

What a week. I felt a little lightheaded on Wednesday, which progressed to dull aches in my bones and a complete loss of appetite by lunch time. By the end of the work day, I had developed a deep, barking cough and a sore throat. Needless to say, my boss kicked me out early, trying to prevent the spread of whatever plague I was incubating in my lungs.

My trusty thermometer showed a slight fever, so I took some generic cough-and-cold medicine, hoping that the decongestant would help the sniffles and cough, and the acetaminophen would fight the fever. I spent the evening on the couch, coughing and sneezing and constantly shifting to get comfortable. My sleep that night was horrible despite the NyQuil I took to knock myself out - the drugs didn't seem to be doing anything at all, and I woke up at 5am with an even higher fever. I called work to tell them I'd be staying home, took more cough-and-cold stuff, and tried to sleep, but the back pain made it impossible to get comfortable. For a while I was concerned it might be my kidney stones coming back to get me, but the pain was in the wrong place for that, thank goodness.

Once the sun was up, I called my doctor's office, and they told me they would be able to squeeze me in that afternoon. In desperation to survive until my appointment, I switched to ibuprofen for the pain - miraculously, it worked. I slept until appointment time.

The official diagnosis? Not strep throat, and not influenza. Bronchitis, likely viral, is what she labeled me with, which is sort of a cop-out, because it just means I have irritated bronchial passages. Um, yeah. I've been coughing my head off for 24 hours. Trust me, if the bronchi weren't irritated, they sure are now. The good doctor told me to pick up some guaifenesin (in some types of Robitussin and Mucinex), then go home and sleep it off.

When I went back to work on Friday and reassured everyone it wasn't the flu, I was asked "how do they know that? Did they test you that quickly?" Yes, yes they did. Thankfully for public health and for pandemic monitoring, hospitals and doctor's offices have a rapid influenza test that can give you an answer within 15 minutes. This makes it easier to distinguish "just a cold" from "influenza", and track the spread of seasonal or pandemic flu.

How does the test work?

Rapid flu test, image from bd.com

First, a swab is stuffed up the patient's nose. Usually, it's a long Q-tip type swab, and it gets rotated in the nostrils a few times before being put back into its holder for the lab. The throat can also be swabbed, because flu viruses hang out in both areas. When the H1N1 influenza, or swine flu, was spreading, it was discovered that the highest concentrations of viruses could be found high in the nasopharynx, or the place where your nasal passage meets your throat, way up past your palate. That's a very difficult area to reach, so a different, flexible swab became the preferred collection method. Here's an illustration:

Nasophagyngeal specimen collection. Image from Stanford University Medical Center.

Yes, it's about as unpleasant as it looks. I can testify to that.

Once the swab gets to the lab, it's dipped into a cocktail of liquid reagents that will extract the flu viruses from the swab, and then the liquid is placed into small wells at the end of a test card. As the liquid moves across the card by capillary action, it passes through a zone, right near the wells, where antibodies against influenza antigens (particles) are embedded. The antibodies are tagged with colored molecules, so as you watch the liquid move across the card, it will look pinkish, or blue, depending on which manufacturer's kit you're using. The antibodies get dragged along whether or not they've bound any antigen - that's why every test has a control line and a test line, to be sure the antibodies are there and moving properly. 

The test line is a strip of the card coated with antibodies against the first antibody, so as the colored antibodies get dragged along, they will stick to that area. When enough of them congregate there, you see a colored line in the control area, meaning that the capillary action is effective and the test is working properly. The control line is a little different - it's got a set of antibodies against another part of the influenza virus, so if those color-tagged antibodies picked up any flu viruses at the start, this is where they'll get caught up, and make a colored line in the test area. If there are no flu antigens in the specimen, the liquid will pass on through and no labeled antibodies will bind here, so there won't be a line.

Almost every rapid test works in a similar way. Pregnancy tests, Strep tests, anything that involves a strip dipped into a liquid specimen, or drops on a card - they all work on the same principle. The antigen, whether it's a virus (flu, mono), bacterium or bacterial toxin (Strep, C. difficile), or hormone (hCG for pregnancy tests, LH for ovulation tests), travels along a card and gets picked up by labeled antibodies, and then trapped along a test line so we can see it.

Isn't science great?

Now if only they could cure non-influenza viral respiratory infections with a magic pill!