What Are Creatinine Tests?

Updated on 2023-12-24 15:52:25 15 11

What to expect when undergoing the tests

By Debra Manzella, RN

Updated on July 28, 2023

Medically reviewed by Jamin Brahmbhatt, MD

Fact checked by Shereen Jegtvig

Creatinine tests are used to measure the level of a waste product, known as creatinine, in your blood and urine. Creatinine is produced during the normal breakdown of muscle tissue, and because it is produced at a relatively steady rate, labs can use it as a marker to see how well your kidneys are functioning. Creatinine tests are used to screen, diagnose, and monitor kidney (renal) disorders, including chronic kidney disease and acute renal failure. They can also be used to evaluate how other diseases, such as heart and liver disease, impact the kidneys.¹

Illustration by Emily Roberts, Verywell

Purpose of Test

Creatinine tests are typically used to diagnose or rule out kidney disease and are ordered if your symptoms are suspicious. The tests may also be used to see if your kidneys are being damaged by another disease (such as heart failure) or medical treatment (such as exposure to certain chemotherapy drugs). Creatinine tests may also be used to assess your response to kidney treatment or to check that your kidneys are healthy prior to surgery.¹

Creatinine tests are among the first tests a healthcare provider will turn to evaluate kidney function. They are fast, easy to use, and relatively inexpensive to perform. They can be used as much for a routine health exam as for the diagnosis and monitoring of a disease.

How the Test Works

The body continuously produces creatinine in the course of everyday functioning. It is created when a substance called creatine, found primarily in muscles, is converted into energy. Creatinine is a by-product of that process.

If your kidneys are functioning normally, creatinine will be excreted from the body in urine. If the kidneys are impaired, creatinine cannot be excreted as effectively and will begin to accumulate in the bloodstream.

Two different creatinine tests are used to determine the levels in your urine and blood:¹

Serum creatinine (SCr), a blood test, reveals how much creatinine is circulating in the bloodstream. Since the rate of production and excretion is relatively steady, any increases above the expected range can be considered a reliable indication of renal impairment.
Creatinine clearance (CrCl), a urine test, compares the SCr with the amount of creatinine excreted in urine over 24 hours.

Serum creatinine output is proportional to muscle mass, meaning that people with more muscle will have higher SCr readings. Similarly, strenuous physical activity can affect both SCr and CrCl readings, given than exercise results in more creatine being converted into energy.²

Indications for Testing

Serum creatinine is included as part of a standard battery of tests known as the basic metabolic panel (BMP), which includes albumin, blood urea nitrogen (BUN), calcium, electrolytes (sodium, potassium, chloride, and bicarbonate), glucose, and phosphorus. The BMP can be used for screening purposes or to help diagnose a suspected kidney disorder.³

Creatinine clearance was once commonly used for screening purposes but, due to the need for a 24-hour urine collection, has largely been supplanted by a newer algorithm—the eGFR—which only requires the SCr.

With that being said, creatinine clearance is still routinely used for diagnostic purposes, especially when a 24-hour urine volume test is needed for more comprehensive evaluation. The test may also provide more accurate insights if you have especially large muscle mass or a marked loss in muscle mass.⁴

Conditions it Helps Diagnose

The creatinine tests can be used help diagnose any number of kidney disorders, broadly described as either a chronic kidney disease or an acute kidney injury.

Chronic kidney disease (CKD) describes the progressive and non-reversible impairment of renal function. It is most commonly caused by three conditions—diabetes, hypertension, and glomerulonephritis—but can also be caused by lupus, polycystic kidney disease (PKD), and recurrent kidney infections, among others.⁵

In some cases, CKD can lead to end-stage renal disease (ESRD) in which dialysis or a kidney transplant is needed to prevent death.⁶

Acute kidney injury (AKI) refers to any condition that harms the kidneys and leads to impairment. Unlike CKD, an AKI is sometimes reversible.⁷ An AKI may be related to an injury or disorder situated before the kidneys (prerenal), in the kidneys (renal), or after the kidneys (postrenal).

Among the examples:⁷

Prerenal causes include dehydration, heart attack, major organ failure, sepsis, significant blood loss, or the overuse of nonsteroidal anti-inflammatory drugs (NSAIDs.
Renal causes include kidney cancer, kidney trauma, major kidney infection, allergic drug reaction, or vasculitis.
Postrenal causes include kidney stones, blockages in the ureters (tubes draining kidneys), urinary retention, urologic cancer (such as bladder cancer or prostate cancer), or any condition that can compress a ureter from the outside.

An AKI can also lead to acute renal failure, which in some cases can be reversed once the underlying injury or disorder is treated.⁸

Risks and Contraindications

Creatinine tests are safe and minimally invasive.

While there are no contraindications for either test, the creatinine clearance test may be impractical for people with acute urinary retention (the decreased ability to urinate).

Before the Test

There are no preparations needed for a serum creatinine blood test. The 24-hour urine collection, by contrast, may require you to stop certain medications or activities that may interfere with the results.

Timing of Blood Test

You can take the serum creatinine blood test at any time of the day. If a creatinine clearance urine test is ordered alongside the serum creatinine, more planning is involved.

Timing of Urine Test

For a creatinine clearance urine test you need to get a special container from the lab to take home with you. Once the urine collection is complete, you must return the container as soon as possible the next day.

When picking up the collection kit from the lab, you will need to provide your ID and personal information so that a label with a generated code can be printed and placed on the collection container. Your insurance or payment details may also be taken so that you can drop the sample off at the lab without delay.

It is often best to perform the 24-hour urine collection on your day off so that you don't have to transport the container from one place to the next. For schoolchildren, you may want to start the collection on a Sunday morning and drop the sample off at the lab on Monday morning.

Location

The blood tests can be performed at a healthcare provider’s office, clinic, hospital, or independent lab facility. Some walk-in facilities do not require an appointment.

The urine collection is performed at home. You will need to have a cool place, like a refrigerator or a portable ice chest, to keep the sample cool.

What to Wear

For the serum creatinine blood draw, either wear short sleeves or a top with sleeves that can be easily rolled up.

Food and Drink

Food restrictions are generally not required for either test. However, for the 24-hour urine collection, you may be asked to avoid alcohol the day before and during the test. You may also be asked to avoid strenuous exercise, which can affect your results.

Medications

While you generally do not need to stop taking medications for a serum creatinine blood test, you may be advised to do so for the 24-hour urine collection. This is because the aim of the urine test is to get an accurate representation of how your kidneys function devoid of any substances that influence filtration.

As such, it is important to advise a healthcare provider about any drugs you may be taking, whether they be prescription, over-the-counter, nutritional, traditional, or recreational.

Problematic drugs may include:⁹

Antacids like Tagamet (cimetidine)
Antibiotics like Bactrim (trimethoprim-sulfamethoxazole)
Anticonvulsants like Phenurone (phenacemide)
Calcitriol (biologically active vitamin D)
Corticosteroids
Salicylates, including aspirin

With that being said, never stop taking any medication without first speaking with a healthcare provider.

What to Bring

Be sure to bring your ID and health insurance cards with you when signing in at the lab.

Cost and Health Insurance

The cost of a serum creatinine blood tests will vary depending on the lab and its location. While insurance pre-authorization is generally not required, you may want to call in advance to find out how much your copay or coinsurance costs will be.

If you are uninsured, shop around for prices. Generally speaking, independent labs have lower costs. Some of the larger labs may even offer tiered pricing if you meet certain income qualifications. Ask about patient assistance programs designed to help low-income families.

Other Considerations

If your child is undergoing the 24-hour urine collection, take extra care to explain the reasons for the test and why the samples cannot be missed. Younger children in particular can get easily distracted and forget.

If you turn the procedure into a game rather than a chore—rewarding the child with a gold star and grand prize for not missing a collection, for example—you may provide the incentive needed to keep your him focused.

During the Test

You can either perform the urine collection first and have the blood test done when you drop off the sample or vice versa.

Blood Test

Pre-Test: On the day of the test, after signing in and confirming your insurance information, you will be taken to an examination room where your height and weight will be recorded. This will later be used to calculate your body mass index (BMI) for the eGFR. Other information will also be collected and will also factor into the analysis.

Throughout the Test: The blood draw, done by a phlebotomist, is performed as follows.

You will be placed in a chair with an elevated armrest and asked to roll up your sleeve. The phlebotomist will often check to see which arm has the fuller vein.
An elastic band, called a tourniquet, is placed around your upper arm to help the vein swell.
The skin is then wiped with an antiseptic swab.
A needle is gently inserted into the vein. You may feel a little pain or pressure. Let the phlebotomist know if it is causing you any significant pain.
Blood is extracted into a vacuum-sealed test tube; the needle, and then the tourniquet, are then removed,
Your arm is bandaged and the sample is sent to the pathologist for evaluation.

Post-Test: You will be free to go unless you feel lightheaded from the blood draw, in which case you may be asked to rest until you feel well.

24-Hour Urine Collection

Pre-Test: Gather the collection supplies provided to you by the lab. You should have one or more large containers with removable lids, which may or may not have a preservative agent inside them, and a small plastic collection cup. You will should also have a preprinted form to record the time and date of each urination.

Unless a healthcare provider tells you otherwise, the 24-hour collection will not begin until the morning. It is important to remember that you cannot miss a sample because the urine volume plays as much a part in the CrCl and eGFR calculations as the concentration of creatinine.

Throughout the Test: To collection procedure can vary slightly but generally involves the same basic steps.

When you wake up in the morning, you will not save the urine from your first urination. Flush this first specimen, but take a note of the date and time. This is your start time.
From then onward, use the plastic collection cup to catch all the urine you pass for the next 24 hours. You can also use a glass or paper cup, but not a metal one.
After each urination, pour the sample into the collection container with the lid. Rinse the collection cup after each use. Throw away a paper cup, if you use one, after a single use.
Store the urine in the refrigerator or ice chest unless instructed otherwise.
Record the time of each urination.
To finish the process, try to urinate at the same time the next day, 24 hours after the start time. If you can't, that's OK. Record the final (stop) time and date.

Post-Test: Once the collection process is complete, the sealed containers need to be taken to the lab as soon as possible. If you are delayed for any reason, call the lab for instructions.

After the Test

Following the serum creatinine blood draw, you may have bruising or localized swelling at the puncture site. If you experience excessive pain or have signs of infection (including fever, shivers, rapid heart rate, or rapid breathing), call your practitioner immediately. Infection, however, is uncommon.

There is no special care needed after a 24-hour urine collection. Unless a healthcare provider tells you otherwise, you can restart any medications you have stopped and return to your normal daily activities.

Interpreting the Results

The results of the creatinine tests can provide a very strong indication of how well your kidneys are functioning. The results are classified based on a reference range (RR) which delineates the range of numeric values considered normal for that test.

Anything above the upper end of the RR is considered abnormally high (often described on the report with the letter "H"). Similarly, anything below the lower end of the RR is considered abnormally low (described with the letter "L").

The normal ranges for a serum creatinine (SCr) test are:¹⁰

BMJ Best Practice. Assessment of elevated creatine.

0.6 to 1.1. milligrams per deciliter (mg/dL) for females
0.7 to 1.3 mg/dL for males

The normal ranges for a creatinine clearance (CrCl) test are:¹¹

70 to 130 mL/minute for children
85 to 125 mL/minute for females
75 to 115 mL/minute for males

The CrCl range in children is not so straightforward. It relies on one of several different algorithms that calculate the CrCl based on age, weight, height, and SCr.

Normal value ranges may vary slightly among different labs. Some use different algorithms or test different samples. As such, you should try to use the same lab whenever monitoring kidney function to ensure accurate comparative results.

Follow-Up

Follow-up testing may be needed to determine the underlying cause for an abnormal reading. While much of the focus would be placed on the kidneys, the problem may be related to an entirely different organ system that either directly or indirectly impacts renal function. Ultimately, as part of an interrelated system, any disease that affects one organ will affect others to some degree.

Generally speaking, if your creatinine test results are high, you may have:¹²