Most people are aware, for example, that several medications taken together can sometimes cause harmful interactions. Most also know that drug allergies can pose significant hazards. (These are topics of other articles in this series). And, many people know that young children, elderly adults, pregnant women, nursing women, and severely debilitated people may all be at higher risk for adverse drug events.

But what most people don’t know is that a simple blood test can be one of most important pieces of information in determining correct dose of many medications…and results of that test are almost never available to your pharmacist, especially if your pharmacist fills your prescription in a retail drugstore. (And, that’s a bummer.)

The test I’m talking about, of course, is “serum creatinine test” (“SEAR-‘em cree-AT-tuh-neen tehst”. It’s a difficult name to pronounce and a difficult test to understand…but one that you need to know about if you or loved ones are taking over-the-counter or prescription medications.)

A serum creatinine test gives a physician or pharmacist an estimate of kidney function.

Serum creatinine is “bean counter” of modern medicine…letting folks know if beans (the kidneys) are working as well as they should.

Why is that important?

Well, kidney function is extremely important in determining correct doses of many medications since kidneys (along with liver) assist in removal of medications from body.

Almost all medications (and/or their chemical by-products) are either removed by liver, by kidneys, or, in many cases, by both kidneys and liver working together.

So, simply put, when beans are not working well, many medications will accumulate in body and increase risk of drug side effects. And that’s an even bigger bummer. (The same is true for liver problems, and we will talk about that in another article in this series.)

As a result, patients with reduced kidney function often need LOWER doses of many medications.

So how does this all work?

Well, creatinine is a chemical that occurs and circulates naturally in human body. It is result of normal protein break-down. And, like many medications, creatinine is normally removed from blood by beans. So, when kidneys are not working well, level of creatinine circulating in bloodstream will start to go up…just like blood level of many medications.

Physicians and pharmacists are routinely and easily able to determine how much creatinine is in blood with results of a serum creatinine test. (This test is part of a very common panel of blood tests. And, if serum creatinine is high, many drugs need to have a lower dose.) The normal value for serum creatinine is about 0.4 to 1.5 mg/dl…but that can vary a bit from lab to lab. So remember: “kidneys no work…serum creatinine go UP”.

Now, serum creatinine is not best measure of kidney function (there are other tests that are much more accurate), but results of serum creatinine test are usually most readily available…and cheapest…and are generally accurate enough for most purposes…so serum creatinine is de facto standard for estimating kidney function…most of time. The gold standard test that doctors use for measuring kidney function is called "creatinine clearance" (cree-AT-tuh-neeeen CLEAR-uhhh-nce) However, not many patients get this particular test because it is pretty darn inconvenient…and smelly. You have to collect all of your urine for 24 hours and keep it in fridge. Not a lot of volunteers for this test….

Creatinine clearance is volume of blood that kidneys clear of creatinine in a given amount of time (and it is usually reported as milliliters per minute).

So, when kidney function decreases, creatinine clearance (the amount of blood that kidneys are successfully “cleaning”) also decreases.

So remember: “kidneys no work…creatinine clearance go DOWN”. (Note: this is easy to remember because it is exact opposite of what you were initially thinking, and opposite of what happens with serum creatinine. Most of medicine is like this.)

Now for super tricky part just for those gunning for an A. There is a way to “guestimate” creatinine clearance using serum creatinine…isn’t that neat. And, that’s probably best way to determine renal function if a measured creatinine clearance is not available.

What you do is run serum creatinine value through a fancy equation that will give you an estimated creatinine clearance, which is itself an estimate of kidney function. (Estimates of estimates of estimates…that’s kind of exacting science I live for.)

Yesterday, I picked up a new antibiotic prescription for my daughter from my local pharmacy.

(We recently adopted my daughter from India where she had recurrent ear infections resulting in severe hearing loss. And, she is about to undergo second of several planned surgeries in order to try to repair damage.)

Before putting her to sleep, I got new medication out of bag, glanced at instructions, and prepared to give her drug according to instructions on label.

Just before doing so, I had a quick double-take.

Something seemed to be wrong. I looked at instructions again, and thought to myself slowly, “What’s going on…this doesn’t seem right.” Then, it hit me that dose seemed awfully high for her.

It took me a minute or two to put pieces together (it had been an unusually tough fight getting her ready for bed, I was tired, I was confident in my daughter’s physician, and I was thinking perhaps less critically that I should have). And then I noticed it. The label had a stranger’s name on it.

After another moment or two, I saw what had really happened.

The medication came in a box. Each side of box had a different label...one label was for my daughter and one label was for a stranger. And, stranger’s dose was more than double what my daughter’s surgeon had recommended.

(This error didn’t happen in a mom-and-pop pharmacy. It happened in a modern new chain pharmacy whose name you would recognize from advertisements on TV.)

I’m not a surgeon…and I’m not a pediatrician…but I am a physician trained in internal medicine and I have spent most of last twelve years writing about, speaking about, and developing systems to reduce frequency of medication error and improve safety of pharmacy practice.

This pharmacy error brought topic of drug safety home to me…literally.

What I can tell you is that this sort of error occurs all too often in United States (and around world). And, that it can have devastating consequences for people involved.

A recent study in New England Journal of Medicine indicated that 25% of patients who take one or more prescription medications will experience an adverse drug event within three months—and 39% of these are preventable or avoidable.

The Harvard Medical Practice Study found reported in JAMA in 2001 that 30% of patients with drug-related injuries died or were disabled for more than 6 months.