A 24-hour urinalysis is a timed urine collection used in the metabolic evaluation of urinary stone disease, proteinuria evaluation, and estimation of renal function via creatinine clearance, estimating residual renal function in end-stage renal disease with urea and creatinine clearance. The testing is usually performed in an outpatient setting while the patient consumes their usual diet. Results are combined with detailed medical and dietary history, serum chemistry, and stone composition to guide prophylactic stone-reducing treatment. A 24-hour urine study can also be used in the pediatric population when inherited conditions such as primary hyperoxaluria and cystinuria are involved. [rx][rx]
A 24-hour urine collection is a simple lab test that measures what’s in your urine. The test is used to check kidney function. A 24-hour urine collection is done by collecting your urine in a special container over a full 24-hour period. The container must be kept cool until the urine is returned to the lab.
Alternative Names
- Urine protein – 24 hours;
- Chronic kidney disease – urine protein;
- Kidney failure – urine protein
Indications of 24-Hour Urinalysis
- Diabetic nephropathy. This happens when someone has uncontrolled diabetes. It causes high levels of protein (albumin) in the urine and can lead to kidney damage.
- High blood pressure. Abnormally high blood pressure can lead to lasting (permanent) kidney damage.
- Lupus nephritis. Lupus is an autoimmune disease where the immune system attacks the kidneys and damages them.
- Frequent urinary tract infections.
- Prolonged urinary tract blockage.
- Alport syndrome. This health problem causes vision and hearing problems, as well as progressive scarring of the kidneys. The syndrome is passed down through families.
- Nephrotic syndrome. This health problem has several different causes. Symptoms include protein in the urine, low protein in the blood, high cholesterol levels, and tissue swelling.
- Polycystic kidney disease. This health problem causes the growth of many fluid-filled cysts in the kidneys. This makes the kidneys larger. Over time, it takes over and destroys working kidney tissue.
- Interstitial nephritis or pyelonephritis. This is an inflammation in the small structures in the kidney. It’s often caused by infection.
- Screening for preeclampsia in pregnancy. Preeclampsia is a dangerous health problem that sometimes occurs in pregnancy. It causes high blood pressure and can lead to organ failure.
- Kidney stones (nephrolithiasis). Kidney stones form because of an imbalance of minerals, salts and water in the urine.
There may be other reasons for your healthcare provider to recommend 24-hour urine collection.
What are the risks of a 24-hour urine collection?
A 24-hour urine collection is a safe, easy test. People can collect urine on their own. Certain factors may affect the accuracy of a 24-hour urine collection. These include:
- Forgetting to collect some of your urine
- Going beyond the 24-hour collection period and collecting too much urine
- Losing urine from the specimen container through spilling
- Not keeping urine cold while collecting it
- Acute stress
- Vigorous exercise
- Certain foods, such as coffee, tea, cocoa, bananas, citrus fruits, and vanilla
There may be other risks depending on your specific health problems. Be sure to discuss any concerns with your healthcare provider before the collection.
How do I get ready for a 24-hour urine collection?
- Your healthcare provider will explain the procedure and you can ask questions.
- Make sure you understand if you need to stay away from certain foods while collecting your urine.
- You will be given large containers to store your urine and a container to urinate into. Make sure you know how to use them. Have a cold place to store the urine while you’re collecting it. For instance, a refrigerator or in a cooler on ice.
- You may be told to start the collection at a specific time.
- If possible, choose a 24-hour period when you will be at home so you do not have to transport your urine.
- If you are pregnant or think you may be, tell your healthcare provider.
- Make sure your provider has a list of all medicines (prescription and over-the-counter), herbs, vitamins, and supplements that you are taking.
Based on your health condition, your healthcare provider may request other specific preparation.
What happens during a 24-hour urine collection?
A 24-hour urine collection may be done on an outpatient basis. This means you go home the same day. Or it may be done during a hospital stay. Procedures may vary depending on your condition and your healthcare provider’s practices.
Generally, a 24-hour urine collection follows this process:
- You will be given 1 or more containers for collecting and storing your urine. A brown plastic container is typically used. A special pan that fits in the toilet or a urinal may be used to collect the urine. You will need to transfer the urine from the collecting container to the storage container. You will need to keep it cold.
- The 24-hour collection may start at any time during the day after you urinate. But your healthcare provider may tell you when to start. It is common to start the collection the first thing in the morning. It is important to collect all urine in the following 24-hour period.
- Don’t save the urine from your first time urinating. Flush this first specimen, but note the time. This is the start time of the 24-hour collection.
- All urine, after the first flushed specimen, must be saved, stored, and kept cold. This means keeping it either on ice or in a refrigerator for the next 24 hours.
- Try to urinate again at the same time, 24 hours after the start time, to finish the collection process. If you can’t urinate at this time, it is OK.
- Once the urine collection has been completed, the urine containers need to be taken to the lab as soon as possible. If you are doing the urine collection at home, you will be given instructions on how and where to take it.
- Depending on your specific health problem, you may be asked to repeat the collection over several days.
Specimen Requirements and Procedure
Instructions for collecting a 24-hour urine sample vary by the laboratory. Typically, the patient’s first voided morning urine is discarded. Subsequent urine produced for the next 24 hours including the next morning’s first voided specimen, is collected in containers that are provided by the laboratory. A preservative solution is added to the urine collection to stabilize the sample for later analysis. Once a full 24 hours of urine is collected, the total volume is recorded. A representative sample from the total collection is then submitted to the laboratory for analysis. Serum samples, usually calcium, potassium, uric acid, and phosphorus, are sometimes also included in the study. It is important for patients to adhere to their normal diet and activities during the collection.[rx][rx]
Once the analysis is complete, a detailed report of the results is provided to the ordering clinician. These results are used to direct prophylactic medical management. Collecting a sample for a full 24 hours can be difficult for some patients and is certainly inconvenient. However, it is necessary to accurately and reliably identify urinary chemistry risk factors for calculus formation as spot urine chemistry is inadequate.
A chemical composition analysis of any stone material is very helpful if available.
Diagnostic Tests
Various labs offer 24-hour urine testing which provides clinicians a detailed laboratory report stratifying stone risk based on the laboratory data points. Typically, 24-hour urine tests for nephrolithiasis prophylaxis will include urinary volume, pH, calcium, citrate, magnesium, phosphate, sulfate, oxalate, and uric acid. Supersaturation ratios for various stone types can then be calculated. In patients with a history of cystine stones or a positive cystine cyanide test, 24-hour cystine levels can also be measured.[rx][rx]
Finding or selecting a laboratory for processing 24-hour urine chemistries can sometimes be challenging. Optimally, all the testing is done in a single laboratory, and the results are presented clearly on just 1 or 2 pages. The 24-hour totals and the relative concentrations should both be given. Be aware that “normal” values are not necessarily “optimal” values for urinary chemical constituents. Optimal urinary chemistry reference values are not reported which makes interpretation a little more complicated. Try to use a laboratory that performs a lot of 24-hour urine testing and reports all the results together. When multiple reports from several laboratories have to be combined to retrieve all the data, it is far more difficult to correlate and analyze.
Key Components of The 24-hour Urinalysis and Their Importance.
Urine Volume and Creatinine
Decreased urine volume is a major risk factor for stone disease as concentrated urine raises the supersaturation of all stone-forming salts. A prospective trial by Borghi et al. in 1999 helped define a goal urinary volume level of 2500 mL per day to reduce stone risk. Furthermore, urine volumes over this amount can decrease stone risk even further.
Urine creatinine excretion is used to determine the accuracy of a timed urine collection. As a byproduct of muscle metabolism, the excretion of creatinine is relatively stable based on muscle mass. The average daily excretion of creatinine for males is 18 to 24 mg/kg and 15 to 20 mg/kg for females. Thus, a lower than expected creatinine excretion suggests an incomplete collection.
pH
Human urine has a pH typically between 4.5 and 8.0. Urine pH is a critical data point as changes in urine pH can drive the crystallization of certain salts. Crystallization of calcium phosphate, calcium oxalate, uric acid, cystine, and struvite are all pH-dependent. Calcium oxalate precipitation is typically not as pH-dependent as the others. Uric acid stone risk is greatest in the acidic range below 5.5. Calcium phosphate crystals form in an alkaline environment of 6.5 and above. Average urine pH over a 24-hour period should fall between 5.7 to 6.3, which limits pH-dependent stone formation.
Sodium and Potassium
Urinary sodium excretion roughly equates to dietary sodium intake. As urinary sodium increases, urinary calcium excretion increases. Because of this relationship, control of dietary sodium is key to controlling hypercalciuria. Lower sodium diets typically allow for up to 1500 mg of dietary sodium per day. Urinary potassium concentration is most useful in monitoring compliance of treatments such as potassium citrate. Potassium citrate supplements should result in marked increases in urinary potassium secretion.
Magnesium
Magnesium is an inhibitor of urinary crystallization thus decreasing stone risk. Roughly half of the dietary magnesium is excreted in the urine. Low urine magnesium is typically dietary in origin.
Calcium
Elevated urinary calcium concentration can be found in nearly half of patients forming calcium stones. Urine calcium concentration is dependent on dietary calcium, sodium intake, and protein intake. Moderate calcium intake is typically recommended to limit urinary excretion while maintaining bone health. Diets low in calcium can be lithogenic, due to increased oxalate absorption in a low calcium diet. Modulation of urine calcium is often accomplished with diet changes or medications depending on etiology.
Citrate
Citrate is a potent inhibitor of calcium salt crystallization. Hypocitraturia is a common risk factor for stone disease and can be found in up to a third of calcium stone formers. Low urinary citrate can be from a variety of factors including diet, metabolic acidosis, or hypokalemia. Hypocitraturia can also be idiopathic. Citrate can be found in foods such as citrus juice. Most patients with low urinary citrate require supplementation as dietary means alone is insufficient.
Concentrated citrate supplements such as potassium citrate are commonly available. Optimal urinary citrate levels are roughly 300 mg per 1000 mL of urine. Low urinary citrate levels in the setting of thiazide therapy may correlate with hypokalemia. A 24-hour urine study is used to monitor urinary citrate concentration and resultant urinary pH level. Over alkalinizing, the urine can predispose to calcium phosphate stones if the pH consistently exceeds 7.0.
Oxalate
High urine oxalate is another common abnormality in the urine of calcium stone formers. Roughly a third of calcium stone formers will have elevated urine oxalate. Oxalate is both endogenous and dietary. Dietary oxalate is absorbed in the colon and distal portions of the ileum. Normal oxalate excretion ranges from around 40 to 50 mg per day. Reductions in excretion can have goals as low as 25 mg per day. Dietary sources of oxalate include black tea, nuts, chocolate, and green leafy vegetables like spinach. Excessive vitamin C supplements are also metabolized to oxalate in the urine. For this reason, vitamin C supplements should be limited to 1000 mg or less daily. Enteric hyperoxaluria can be a significant risk factor for patients with inflammatory bowel disease, cystic fibrosis, pancreatic insufficiency, or previous bariatric bowel surgery.
A more detailed review of 24-hour urine chemistry interpretation and treatment guide for kidney stone prevention can be found in our companion review article 24-Hour Urine Testing for Nephrolithiasis: Guide to Interpretation by Leslie and Bashir.
Results, Reporting, Critical Findings for 24-Hour Urinalysis
Components of 24-hour urine exams vary by the laboratory. Components included in most standard 24-hour analyses include urine volume, the concentration of urine calcium, oxalate, citrate, and uric acid, urine pH level, and supersaturation values. Supersaturation of calcium oxalate, calcium phosphate, and uric acid are commonly reported. Other analytes include urine potassium, magnesium, phosphorus, ammonium, chloride, sulfate, and nitrogen in the form of urea. Reports typically include reference range values that help stratify the risk of stone formation. Specialized testing is also available for pediatric patients and patients with cystinuria. These tests include cysteine excretion, supersaturation, and urine pH. The interpretation of urine chemistry requires reference ranges. Urine chemistry is a continuous variable making the strict cut-off points and abnormal values somewhat arbitrary. As urinary constituents reach outside of normal or optimal ranges, the lithogenic risk increases.[rx]
What do the test results mean?
Test results should be available after a couple of days, depending on the lab schedule. A normal test result shows less than 150 milligrams of protein per day. Test results may vary slightly between laboratories. Ask your doctor about the exact meaning of your test results.
Protein in the urine may signify kidney damage or disease. Protein levels may also rise temporarily due to factors such as infection, stress, or excess exercise.
If the protein is caused by kidney damage, the test results will help to determine the extent of that damage. The protein amount can also be used to monitor any disease progression or measure your response to therapy.
Proteinuria is associated with many other conditions. These include:
- amyloidosis, an abnormal presence of amyloid proteins in organs and tissues
- bladder cancer tumors
- congestive heart failure
- diabetes
- urinary tract infection
- use of medications that damage the kidneys
- Waldenström’s macroglobulinemia, a rare plasma cell cancer
- glomerulonephritis, inflammation of the blood vessels in the kidneys
- Goodpasture syndrome, a rare autoimmune disease
- heavy metal poisoning
- hypertension
- kidney infection
- multiple myeloma, a cancer of the plasma cells
- lupus, an inflammatory autoimmune disease
- polycystic kidney disease
Your doctor may order more tests to make a diagnosis.
References