Which of the following may cause a false-positive protein reading on a urinalysis dipstick?

urine testing:

Urine dipstick testing is often done at the bedside as a screening test by the clinician, but the results must come from the laboratory for them to be placed in the chart as a formal test. There are numerous false-positive and false-negative dipstick tests, and the actual value of dipstick testing is likely overrated by clinicians.

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• Urinalysis: A Comprehensive Review
• CME for InFocus!
• What can cause a false positive protein test?
• Which of the following can give a false negative urine protein reading?
• Which of the following can cause a false positive blood on the reagent strip?
• How accurate is dipstick urine test for protein?

Figure

Urine dipstick testing is often performed at the bedside by emergency physicians as an initial evaluation, but it is also a test that must be confirmed by the laboratory. As an overview, dipstick testing has its limitations but is quite helpful, serving as a screening test for some conditions and a more definitive test for others. In complicated cases or those of serious disease, dipstick testing must be correlated with microscopy and clinical parameters.

Urinalysis: A Comprehensive Review

Simerville J, et al.

Am Fam Physician

2005;71(6):1153

https://bit.ly/3jF8Lgp

This review discusses the correct method for performing a urinalysis and highlights the importance and diagnostic value of a number of results found on the dipstick. Visually reading and recording the results of the urine dipstick is no longer adequate or proper. A machine in the lab is used to read the dipstick and print out the results. Quality assurance is very problematic unless this route is used.

Physical Properties: A variety of foods, medications, metabolic products, and infections can cause abnormal urine colors and odors. Normal urine is clear and light yellow in color. Concentrated urine produces a darker color, a common finding in the morning after overnight water restriction. Cloudy urine can be normal, usually caused by precipitated phosphate crystals. Significant pyuria can also cause clouded urine.

Urine clarity is a good but not infallible guide to the presence or absence of a UTI. (Pediatrics. 2000;106[5]:E60; https://bit.ly/30Lg9Qe.) Many believe that odoriferous urine is a sign of infection, but it can simply represent a concentrated specimen or diet. Urine that has prolonged bladder retention can develop an ammonia-like odor. A fecal smell in the urine suggests a GI-bladder fistula. Certain foods, such as asparagus or beets, and a variety of medications can change the odor or color of urine. Myoglobin colors the urine brown, carrots can produce a deep yellow color, and pseudomonas infections, propofol, and amitriptyline may give it a blue or green hue.

Dipstick Analysis: The accuracy of bedside dipstick analysis for hematuria, proteinuria, or urinary tract infection is a subject of much interest and practicality to emergency physicians. Overall, the results of dipstick analysis of urine are not as accurate as thought by many clinicians, and have a number of false-positive and false-negative results. It should also be noted that the commonly used urine dipstick has a finite lifespan, should be kept in a closed container, and should not be constantly exposed to air. Testing with outdated and improperly stored materials can lead to erroneous results.

Urine Specific Gravity: Urine specific gravity (USG) generally correlates with urine osmolality. The most useful information derived from the USG is insight into the patient's hydration status and the concentrating ability of kidneys. The latter function is disrupted in a variety of diseases.

A normal USG ranges from 1.003 to 1.030. USG less than 1.010 is suggestive of relative hydration, and values greater than 1.020 indicate relative dehydration. Pathologic conditions that increase the USG without regard to hydration include glycosuria and the syndrome of inappropriate antidiuretic hormone secretion (SIADH). In such cases, osmolality is the more important parameter to measure.

It should be noted that the purpose of the kidney is to concentrate urine when needed. Many renal diseases alter this concentrating function and result in a fixed specific gravity, about 1.010, the gravity of the glomerular filtrate. This is known as isosthenuria, a condition seen, for example, in patients with renal dysfunction due to sickle cell disease.

The proper way to use a dipstick is to immerse it totally in urine, turn it on its side on filter paper to absorb runoff and keep chemicals from running onto the adjacent patch, and wait two minutes before reading. Don't forget to put the lid back on the container.

Urinary pH: The urine pH generally reflects the serum pH, but the primary and normal function of the kidney is to acidify the urine. Normal serum pH is 7.4, but the normal urinary pH ranges from 4.5 to 8. The generally accepted normal pH of urine is about 5.5 to 6.5. In renal tubular acidosis, the kidney cannot acidify the urine, so the urine can be alkaline while the patient's serum demonstrates a metabolic acidosis.

The urine pH can also be related to diet. Acid urine can be the result of ingestion of fruits (hence the use of cranberry juice for UTI) that acidify the urine. Diets high in citrate and citrus fruits, legumes, and vegetables can produce alkaline urine. Meat eaters tend to have more acidic urine, and vegetarians tend to have alkaline urine. Alkaline urine may suggest infection with a urea-splitting organism (such as proteus) in the presence of a documented UTI. Triple phosphate crystals (magnesium ammonium phosphate) can suggest the presence of a staghorn calculus in alkaline urine.

Hematuria: The strict definition of hematuria by the American Urological Association is the presence of three or more RBCs per high-powered field. The urine dipstick is used to test for the peroxidase activity of erythrocytes, not for the actual presence of the physical RBC. Both myoglobin and free hemoglobin produce a positive dipstick for hematuria because both substances will catalyze this reaction. Myoglobin is the end-product of muscle breakdown. High doses of vitamin C will inhibit this peroxidase activity, and can invalidate the dipstick reading for this test, but vitamin C also produces a false-negative dipstick for glucose, nitrite, and LE. This also holds true for stool guaiac testing; vitamin C can produce a false-negative occult blood in stool. A positive dipstick for blood in the absence of RBCs by microscopy is generally indicative of myoglobinuria or hemoglobinuria, not true hematuria.

The table lists many causes of hematuria. Some rare causes such as Fabry's disease will likely escape the detection and knowledge of the emergency clinician, but it is important to know that hematuria can be associated with malignant hypertension, numerous urinary tract cancers, infections, nephrolithiasis, nephritis (lupus), vasculitis, tuberculosis, and a variety of drugs, including the obvious, heparin and warfarin.

RBC casts are classic for acute glomerulonephritis. Hematuria can also be associated with TTP, renal vein thrombosis, sickle cell trait, or merely running a marathon. Contrary to popular belief, significant hematuria will not elevate the protein concentration to the required cutoff deemed positive, 3 plus or more on the dipstick. The authors note that up to 20 percent of patients with a gross hematuria have a urinary tract malignancy, so unless from a very obvious cause, finding hematuria requires a full workup.

Proteinuria: Healthy kidneys limit the protein permeability of the glomerular capillaries, but diseased kidneys allow more protein to be filtered. Proteinuria is therefore a hallmark of a variety of renal diseases. Blood proteins are normally filtered and then reabsorbed by the proximal tubule cells. Urinary proteins include primarily albumin, but some serum globulins are detected. The actual definition of proteinuria is the excretion of more than 150 mg of protein per day. Patients with early renal disease may have microalbuminuria. Early diabetic nephropathy may not be detected by dipstick testing, so it is not a good screening test for this condition. The dipstick test is sensitive almost entirely to albumin; it will not detect low concentrations of globulins or the Bence-Jones proteins associated with multiple myeloma.

1.Test depends on peroxidase activity of RBS. Tests will be positive with intact or lysed cells. This test is very sensitive and may be positive in normal urine (1-2 RBC/HPF).

The dipstick is actually quite sensitive for proteinuria, and produces positive results by reacting to minor proteinuria that would not be considered clinically significant. Concentrated early morning urine may give the false impression of significant proteinuria. The authors state that the dipstick must be 3 plus or greater for protein to be considered significant. Interestingly, prolonged standing can produce proteinuria, termed orthostatic (postural) proteinuria. Iodinated radiocontrast agents and a highly alkaline urine may turn the dipstick falsely positive for protein.

Glycosuria: Glucose is normally filtered by the glomerulus, but then almost completely absorbed in the proximal tubule. Glycosuria results when the amount of filtered glucose exceeds the kidney's ability to resorb, making it an abnormal finding. The blood glucose usually needs to be at least 180 mg/dL to produce glycosuria and to be detected by the dipstick.

Ketonuria: It is not normal to find ketones in the urine. Ketones are the product of fat metabolism that is commonly encountered in uncontrolled diabetes. Some ketonuria can occur normally in patients on a carbohydrate-free diet (high-protein weight loss diets) and occasionally with starvation or a prolonged fast.

Nitrites: There is a difference between nitrates and nitrites. Nitrites are not normally found in urine, although dietary nitrates are excreted by the kidney. The dipstick will identify this condition when bacteria reduce urinary nitrates to nitrites. Not all bacteria have this function. One needs the presence of appropriate bacteria for the dipstick to register a positive nitrite. The diet must also have sufficient nitrates for a nitrite test to be positive. The urine must remain in the bladder for about four hours for bacteria to produce nitrites.

A positive nitrite test usually means infection. It generally requires more than 10,000 bacteria per mL to turn the nitrite dipstick positive, making it a specific but not a sensitive test. A negative nitrite test does not rule out a UTI, but a positive one strongly suggests infection. Infection with non-nitrate-reducing organisms will result in a negative nitrite test. The nitrite reagent on the dipstick is quite sensitive to environmental air, so this test is the one that is most affected when out-of-date dipsticks or those kept in an open container are used. Improperly stored dipsticks are the most common cause of a false-positive test for nitrites.

Leukocyte Esterase: LE is an enzyme produced by the lysis of neutrophils. It may signal pyuria associated with UTI. WBCs anywhere in the GU tract, including the vaginal vault, will produce LE. It is somewhat nonspecific, and will be positive in patients with chlamydia infections, urethritis, tuberculosis, bladder tumors, viral infections, nephrolithiasis, foreign bodies, and corticosteroid use.

Bilirubin and Urobilinogen: Urine does not usually contain bilirubin. Any bilirubin found in the urine is conjugated bilirubin because unconjugated bilirubin cannot pass through the glomerulus. Biliary obstruction or liver disease will cause an elevated urine bilirubin. There can normally be in the urine small amounts of urobilinogen, the end-product of conjugated bilirubin after it passes through the bile duct and has been metabolized in the intestines. This urobilinogen is then reabsorbed into the portal circulation and eventually filtered by the kidney. Patients with hemolysis or some type of liver disease will have an elevated urobilinogen level. If the bile duct is obstructed, less bilirubin enters the intestine, and therefore less urobilinogen is detected in the urine.

Comment: This article makes a clinician yearn for the memory and recall prowess one had in medical school. One marvels at how the interpretation of the lowly UA dipstick has morphed into a sophisticated science. Take-home points from this discussion are that dipstick testing is not an exact science, there are numerous false-positive and false-negative results, and the actual true value of dipstick testing is probably overrated by clinicians.

Trace-positive dipsticks often confuse clinicians, and those done in the ED don't always match the lab tech's reports. There is no standard totally agreed-upon about how to use the dipstick in the ED. Most clinicians use it to screen for problems, and eschew sending the UA to the lab for repeat testing or microscopy if the dipstick is totally negative. (Clin Nephr. 1994;41[3]:167.) A negative dipstick does not rule out infection. Positive dipstick findings may prompt the lab to perform additional studies. When serious pathology is suspected, however, one usually combines dipstick testing with microscopy and clinical information. Kidney stones, for example, can be associated with a 10-20 percent incidence of a negative dipstick for blood. Don't rule out a kidney stone solely on the basis of a hematuria-negative dipstick. And hematuria is common with endocarditis and aortic dissection.

The dipstick for blood is probably the test result of greatest utility to the EP, but this is a sensitive test that has a number of false-positives. The few RBCs that normally inhabit the urine (2-3) can give a trace reading.

The dipstick does not identify RBCs. It essentially detects the presence of RBC peroxidase activity, whether these cells are intact, or if there is merely free hemoglobin or myoglobin in the specimen. Even if the patient is on heparin or warfarin, gross hematuria should always prompt a consideration for malignancy. This is similar to finding occult blood in a stool sample in a patient on iron or aspirin. Dehydration and exercise will give a false-positive dipstick for true hematuria, and vitamin C (which blocks peroxidase activity), captopril use, a pH less than 5.1, and proteinuria may produce a false-negative dipstick analysis for blood.

I frequently encounter trace to 1 plus protein via dipstick testing. It is rarely of importance in the ED. Trying to track down trace or 1 plus proteinuria is a useless task in the ED and probably in general practice.

With regard to specific gravity, one reason sickle cell patients often go into crisis when there is no good reason to be dehydrated is that they cannot concentrate their urine. Finding an USG of 1.010 in a patient with advanced sickle cell disease does not mean they are well hydrated. They may be quite dehydrated and unable to concentrate their urine. While USG usually corresponds to osmolality, large molecules in the urine, such as glucose or IV dye, can produce large changes in USG with relatively minimal changes in osmolality. It has been shown that there is no clear or consistent relationship between USG and osmolality, so when osmolality determinations are important, an osmometer should be used. (Arch Dis Child. 2001;85[2]:155; https://bit.ly/34xVdNz.)

With regard to urinary pH (normal 4.5-8.0), there are many causes of alkaline urine, and not all patients with this finding have urea-splitting organisms in the urine. The kidney's task is to acidify urine, and normally a serum pH of 7.4 produces a urine pH of about 6.0. Interestingly, in the presence of urinary tract obstruction by a stone, the kidney loses its ability to secrete acid, and obstruction alone can produce alkaline urine. (Pediatr Nephrol. 1988;2[1]:34.) Patients with a significant metabolic acidosis would be expected to produce an acidic urine, usually below 5.0.

In my experience, finding a trace or 1 plus leukocyte esterase (LE) is not confirmation of infection. This is most likely due to contamination. Interestingly, nephrolithiasis alone in the absence of infection can produce a dipstick positive for LE. This may lead the clinician to suspect infection in the stone former when it is not present.

The ability of the dipstick to diagnose a UTI is not as accurate as most clinicians think. Importantly, the absence of nitrites or LE does not rule out a UTI. The relationship of nitrites and LE to infection is quite variable, and results are all over the board in various studies. One meta-analysis found that a positive LE is 80% sensitive and 90% specific for UTI. A positive nitrite alone is only 50% sensitive for UTI but 95% specific. Combining the two positive tests gives a 98% sensitivity for infection. (Lancet Infect Dis. 2010;10[4]:240).

Many organisms are capable of converting nitrates to nitrites, non-nitrate-reducing organisms can also cause negative nitrite results in infections. Of course, patients who consume a low-nitrate diet will not have the nitrate substrate for the bacteria to convert.

The nitrite test is much more specific for infection than the LE, although it takes a while for the bacteria to reduce the nitrates to nitrites. The urine plus bacteria must remain in the bladder for about four hours to produce a positive nitrite test. Importantly, if your dipsticks are scattered around the ED lab in an open container for more than two weeks, about three-quarters of them will give a false-positive result for nitrites. Perhaps the lab is better at protecting dipsticks than the ED, but I rarely see the top put back on the container in my ED's stat lab. Everyone has difficulty with quality control for dipstick testing in the ED. We all now use machines to read the dipstick rather than relying on a nurse's eyes, and a printout has replaced the pen. Nonetheless, our hospital lab often disagrees with the ED reading for leukocyte esterase and the degree of hematuria.

Like most things in medicine, the dipstick urinalysis is not as straightforward as one would like. It is clearly not foolproof nor a gold standard for many things, and its value is likely overrated. A plethora of conditions produce false-positive or false-negative results. It can serve as a useful guide to the emergency clinician as a screening test or occasionally as a diagnostic test, but the dipstick must be correlated with other testing and clinical information.

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Dr. Robertsis a professor of emergency medicine and toxicology at the Drexel University College of Medicine in Philadelphia. Read the Procedural Pause, a blog by Dr. Roberts and his daughter, Martha Roberts, ACNP, PNP, athttp://bit.ly/EMN-ProceduralPause, and read his past columns athttp://bit.ly/EMN-InFocus.