By Katie E. Golden, MD
In the emergency department (ED), it is common to obtain blood tests as part of a patient’s evaluation. A routine part of this analysis is to check kidney function, and we do this by testing for levels of ‘creatinine’ (abbreviated as Cr). It is common for these values to be abnormal, but it is less common for the patient to have a clear understanding of what to do with this information. This article is dedicated to explaining what it means if your testing is abnormal.
How do the kidneys work?
Before I explain how we use lab tests to measure kidney function, let’s take a step back to explain these bean-shaped organs. Kidneys are complicated, sensitive creatures, and understanding how they work is challenging even for the smartest of doctors. It is easily carried away with the kidney’s complicated life force, but let’s just keep it simple.
The kidney’s job is to filter our blood to maintain a balanced level of water and electrolytes in the body, and at the same time remove unwanted substances from the blood. The kidney maintains a perfect balance of electrolytes (like potassium, chloride, sodium, calcium, and magnesium), and removes toxins (medications, drugs, and natural byproducts from our metabolism) so that the rest of our organs can function well. It does this through a very complicated plumbing system that collects all the unwanted materials, combines it with water, and sends it to the bladder to be excreted as urine.
What do we mean by kidney function?
The kidney has many important functions in addition to regulating the chemicals and electrolytes in our blood. It also plays an important role in hormone production, stimulating the production of new red blood cells, and regulation of our blood pressure. When we refer to kidney function, however, we are focusing on how well it is filtering the blood. So let’s get into the relevant blood tests that let us measure that filtration.
How do we measure kidney function?
Similar to the organ itself, measuring kidney function is not straightforward or easy. In an ideal world, we would have a way to directly measure the kidney’s filtering power. Given that would be extraordinarily complicated, we rely on the blood levels of ‘creatinine’ as a marker of how well the kidneys are working.
Creatinine is a waste product that is excreted into the blood by muscle metabolism and protein breakdown. It makes for a useful marker of kidney function given the kidneys predictably remove nearly all of it from the blood (and convert it into urine to be excreted from the body). While there are still many factors that contribute to creatinine levels in the blood (a person’s muscle mass, their level or protein and meat intake, etc), it is our best way to assess kidney filtration. An increased creatinine level in the blood, for example, suggests the kidneys are not performing at 100%.
Are there other ways to test kidney function?
Creatinine is a common and quick way to assess kidney function, but it is important to mention that other blood tests are used to help with this assessment. A few examples include:
- Blood Urea Nitrogen (BUN) – Urea is another waste product in the blood that we can measure to help us determine kidney filtration, but has a tendency to be more variable and less predictable than creatinine.
- Electrolytes – As mentioned, the kidneys are mainly responsible for controlling the levels of sodium (Na), Potassium (K), Chloride (Cl), Calcium (Ca), and Magnesium (Mg) in the blood. The fluctuations in these levels are often due to other factors outside of kidney function, but can also become abnormal when the kidneys are struggling.
- Glomerular Filtration Rate (GFR) – GFR is a very complicated measurement, and is a lab calculation that inputs different values to best estimate kidney function. Given it is a calculated value, however, there are inherent flaws in using it to measure kidney function.
What do I need to do if my blood tests show abnormal results?
Hopefully, our deep dive into kidney function and creatinine helps you to understand that an abnormally high creatinine does not always mean you have ‘kidney failure’ or ‘kidney disease’. Arnold Shwarzenegger, with his big muscles and heavy meat intake, assuredly lives with higher creatinine values than the normal population. Our kidneys can also become a little sluggish after a day of decreased hydration, for example. A high creatinine often corrects itself after a day of staying hydrated and maybe cutting out that Ribeye at dinner. If you leave the ER after being told your kidney function tests were slightly abnormal, the next step is to schedule an appointment with your doctor to recheck your blood tests. And maybe drink a glass of water and eat a vegetable the day before that appointment.
Please do not delay your re-evaluation of your kidney function, and schedule your appointment as soon as you leave the ER. Please seek emergent care if you develop the following symptoms:
- A significant decrease in the amount you urinate each day, or inability to urinate
- Blood in your urine
- A dark, tea-like appearance to your urine
- Pain in your flank (side area) or abdomen
- New and unexplainable lethargy or confusion
These may be signs that your kidney function is further declining, and warrant prompt evaluation.