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Your Kidney Function Really Matters: A Lighter Look at What You Need to Know to Prevent Adverse Drug Events

 by: Timothy McNamara, MD, MPH

When you (or loved ones) are taking prescription or over the counter medications...there is a lot you should be worried about, and a lot that your pharmacist may not be telling you.

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 the most important pieces of information in determining the correct dose of many medications...and the 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 the '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 the 'bean counter' of modern medicine... letting folks know if the beans (the kidneys) are working as well as they should.

Why is that important?

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

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

So, simply put, when the beans are not working well, many medications will accumulate in the body and increase the 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 the human body. It is the result of normal protein break-down. And, like many medications, creatinine is normally removed from the blood by the beans. So, when the kidneys are not working well, the level of creatinine circulating in the bloodstream will start to go up...just like the blood level of many medications.

Physicians and pharmacists are routinely and easily able to determine how much creatinine is in the blood with the results of a serum creatinine test. (This test is part of a very common panel of blood tests. And, if the 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 the best measure of kidney function (there are other tests that are much more accurate), but results of the serum creatinine test are usually the most readily available...and cheapest...and are generally accurate enough for most purposes...so serum creatinine is the de facto standard for estimating kidney function...most of the 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 the fridge. Not a lot of volunteers for this test...

Creatinine clearance is the volume of blood that the 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 the kidneys are successfully 'cleaning') also decreases.

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

Now for the 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 the best way to determine renal function if a measured creatinine clearance is not available.

What you do is run the 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 the kind of exacting science I live for.)

For adults, that equation is the famous 'Cockcroft-Gault equation corrected for ideal body weight and gender'...the equation everyone loves to hate. The Cockcroft-Gault equation (presumably named after Drs. Cockcroft and Gault...or maybe just Dr. Cockroft-Gault, or maybe Lara Croft), is generally considered very reliable since it has never been well validated in young patients, old patients, thin patients, fat patients...basically all the patients it gets used on. So go figure. Double bonus points if you can remember this:

For men, creatinine clearance =
   ((140- Age) ' IBW) / (72 ' SCr)

For women, creatinine clearance =
   ((140- Age) ' IBW ' 0.85) / (72 ' SCr)

Where Age is in years, IBW is ideal body weight in kilograms, and SCr is serum creatinine in mg/dl.

(For the algebraically-challenged and for anyone wanting help in calculating the results of these complicated equations, please visit this creatinine clearance calculator: http://www.medicationadvisor.com/creatinine/creatinine.asp)

Now once you run this a few times, you'll find that creatinine clearance for young healthy people is about 100 ml/min (we'll just leave off the 'ml/min' part from now on).

And, dead people have a creatinine clearance of about 0, depending on how healthy they are.

Everyone else falls somewhere in between.

(Now someone in the back of the room is saying, 'I just ran this on myself and I have a creatinine clearance of 150'. Well aren't you special? In fact, young good-looking people can have creatinine clearances of 130, 140, or more...but it doesn't do a whole lot of good since 100 is perfectly acceptable. In fact, it's just another case of overachiever overkill.)

Now, if someone has a creatinine clearance of 80, that means that they have about...80% kidney function.

And, if someone has a creatinine clearance of 50, that means that they have about...50% kidney function. (Are you following all of this?)

Many drugs that are eliminated by the kidney will require moderate dosage reductions once a patient's renal function is in the 40-60 range.

Patients in the 20-40 range will typically require very large reductions in dose.

And, patients who are in the 'less than 20' range will often need HUGE dosage reductions for medications eliminated by the kidney (or better yet, they'll need to take medications that are removed by some other organ altogether...if such an alternative is available).

One last tidbit to consider. Renal function declines as people age. No getting around that. But, the rate of decline is different for different people. By the time you're 50 years old,

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