Urine Reagent-Strip Chemistry and Confirmatory Testing

Strip workflow and quality control

The product instructions specify how to dip the strip, remove excess urine, time each reaction, store the strips, and perform quality control. Procedures differ among strip brands and automated readers. A routine manual workflow includes the following steps:

1. Test a fresh, well-mixed specimen in a clean, dry container. Allow refrigerated urine to reach room temperature before mixing and testing.
2. Check the strip container, lot, expiration date, reagent-pad appearance, and acceptable control results.
3. Immerse every reagent pad and remove the strip immediately. For visual reading, start timing as the strip leaves the urine, then remove excess urine by the method specified in the product instructions. Fluid carried between pads can contaminate adjacent reactions.
4. Compare each pad with the color chart at its stated time under adequate light, or place the strip in the validated automated reader according to its instructions.
5. Record the result in the units and categories assigned to that strip system. Do not interpret color changes after the reading window.

For the visually read Siemens Multistix 10 SG example, the bottle label assigns 30 seconds to glucose and bilirubin, 40 seconds to ketones, 45 seconds to specific gravity, 60 seconds to blood, pH, protein, urobilinogen, and nitrite, and 120 seconds to leukocyte esterase. Other products follow their own label.^{22 Siemens Healthineers. Multistix 10 SG Reagent Strips. 2017.}

Store reagent strips in their original, tightly closed container and protect them from moisture, heat, and light. Follow the manufacturer's instructions for storage temperature and stability after opening, and follow the laboratory's quality-control plan for control frequency. Negative and positive controls specified by the manufacturer or validated by the laboratory assess the entire testing process. For the Siemens method cited here, water is an unacceptable negative-control substitute because it lacks the ionic and chemical matrix of urine.^{22 Siemens Healthineers. Multistix 10 SG Reagent Strips. 2017.}

When a result is unexpected, check specimen age, temperature, mixing, strip storage, expiration, timing, and control performance. Repeating the same strip method may reproduce the same interference. Select a follow-up test that uses an independent reaction or a more specific specimen.

Acid-base and concentration findings

pH

The pH pad contains acid-base indicators, commonly methyl red and bromthymol blue, that cover a broad range of approximately pH 5 to 9. Results are reported in discrete color steps. Diet, medications, systemic acid-base status, renal tubular function, and bacterial growth can all affect urine pH. Urease-producing organisms and prolonged storage may make a specimen more alkaline.

Urine pH supports the interpretation of crystals and stones and the assessment of renal acidification. Urine pH cannot establish renal tubular acidosis without serum chemistry and clinical findings. A pH meter provides a more precise measurement when the exact pH affects a procedure or clinical decision.^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

A pH meter uses potentiometry, the measurement of electrical potential between an indicator electrode and a reference electrode. The glass indicator electrode responds to hydrogen ion activity. The reference electrode supplies a stable comparison potential. Calibration converts the measured voltage difference into pH units. This electrical measurement differs from matching the strip's indicator color. Mayo's UPH24 method describes a glass electrode, a calomel reference electrode, and a voltmeter. Other meters may use a different reference system.^{33 Mayo Clinic Laboratories. pH, 24 Hour, Urine.}

Specific gravity

The specific-gravity pad uses a polyelectrolyte and pH indicator to estimate ionic strength. As ionic concentration rises, hydrogen ions released from the polyelectrolyte change the indicator color. Results are usually reported in increments of 0.005.

Because the strip responds chiefly to ions, its result may disagree with methods that also detect nonionic solutes. Protein and alkaline pH can also affect some products. Refractometry responds to solute number and molecular mass. Osmolality counts dissolved particles. The Specific Gravity and Osmolality lesson compares these measurements and their interferences.

Protein and albumin findings

Routine protein pad

The conventional protein pad uses the protein error of indicators. At a constant acidic pH, protein accepts hydrogen ions from an indicator and changes its color. Albumin produces the strongest response. Globulins, immunoglobulin light chains, and other low-molecular-weight proteins can be present despite a weak or negative routine pad.

Highly alkaline or heavily buffered urine, prolonged contact between the strip and specimen, quaternary ammonium compounds, chlorhexidine, visible blood, and intense pigment can cause falsely increased reactions in susceptible products. A very dilute specimen may contain clinically important albumin at a concentration below the pad's detection limit. Interpret a trace or 1+ reaction alongside the specific gravity and quantitative albumin or protein results. ^{1,21 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.2 Siemens Healthineers. Multistix 10 SG Reagent Strips. 2017.}

Proteinuria patterns reflect the source and mechanism of protein entry into urine:

Orthostatic proteinuria is increased after an upright period but is normal in a first-morning specimen collected after recumbency. Its significance depends on persistence, the amount and type of protein, urine sediment findings, kidney function, and the clinical setting.

Albumin-to-creatinine ratio

The urine albumin-to-creatinine ratio (UACR) partly corrects for differences in urine concentration. Kidney Disease: Improving Global Outcomes (KDIGO) classifies albuminuria as A1 below 30 mg/g, A2 from 30 to 300 mg/g, and A3 above 300 mg/g. The older term microalbuminuria names the A2 concentration range, a smaller amount of the same albumin molecule. Chronic kidney disease assessment combines UACR with estimated glomerular filtration rate and evidence that an abnormality persists for at least 3 months. When feasible, confirm a random UACR of 30 mg/g or greater with a subsequent first-morning specimen. Exercise, fever, infection, menstruation, and marked hyperglycemia can cause transient increases.^{44 Kidney Disease: Improving Global Outcomes… KDIGO 2024 clinical practice guideline for the evaluation and… 2024.}

Albumin-sensitive strips can screen for moderately increased albuminuria, but their categories and detection limits are product-specific. Confirm a positive screen with quantitative UACR.

Nonalbumin protein

Sulfosalicylic acid (SSA) precipitates a wider range of proteins than the conventional indicator pad, including globulins and immunoglobulin light chains. Follow the laboratory's validated procedure for specimen preparation, reagent concentration, volumes, timing, and turbidity grades. Radiographic contrast material and some medications can also produce turbidity. Use quantitative protein measurement and electrophoretic methods to investigate a clinically important discrepancy.

Bence Jones proteins are monoclonal free immunoglobulin light chains. They may produce a strong SSA reaction with a weak routine protein pad. The historical heat-precipitation test has inadequate sensitivity and specificity for current confirmation. Investigation may include serum protein electrophoresis, serum immunofixation, serum free-light-chain measurement, and urine electrophoresis with immunofixation as clinically indicated.^{55 Strasinger SK, et al. Urinalysis and Body Fluids. 2021.}

Glucose and ketone findings

Glucose

The glucose pad couples two enzyme reactions. Glucose oxidase forms gluconic acid and hydrogen peroxide. Peroxidase then uses the peroxide to oxidize a chromogen. This reaction is specific for glucose. Oxidizing cleaning agents can produce false-positive color. Ascorbic acid, high specific gravity, cold urine, high ketone concentrations, and delayed testing with cellular or bacterial glucose consumption can decrease the reaction in susceptible products.^{1,21 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.2 Siemens Healthineers. Multistix 10 SG Reagent Strips. 2017.}

The renal threshold for glucose varies among individuals and is commonly approximated as a plasma glucose concentration of 160 to 180 mg/dL. Glycosuria can accompany hyperglycemia, pregnancy, proximal tubular dysfunction, renal glucosuria, and sodium-glucose cotransporter 2 inhibitor therapy. Blood glucose and glycated hemoglobin guide diagnosis and routine diabetes monitoring. Urine glucose reflects only the tested specimen.^{66 Sacks DB, et al. Guidelines and recommendations for laboratory analysis in the… 2023.}

In a heated alkaline reaction, copper-reduction tablets detect reducing substances by converting cupric ions to cuprous oxide. Glucose, galactose, fructose, lactose, pentoses, and some drug metabolites can react. Sucrose is nonreducing. A very high reducing-substance concentration can produce a pass-through color sequence. Observe the reaction continuously and read it according to the product instructions. Copper reduction cannot identify which reducing substance is present or confirm an enzymatic glucose result. Specific blood or chromatographic methods have replaced this broad screen in many clinical applications.

Ketones

Urine ketone pads use sodium nitroprusside to detect acetoacetate. Formulations that include glycine also increase acetone reactivity. The pad does not detect beta-hydroxybutyrate, often the predominant ketone body in diabetic ketoacidosis. Early ketoacidosis may therefore produce a urine result that understates the ketone burden. During treatment, beta-hydroxybutyrate is converted to acetoacetate, so urine ketone color may increase as the patient's metabolic condition improves. Direct blood beta-hydroxybutyrate is preferred for diagnosis and serial assessment of diabetic ketoacidosis when available.^{6,76 Sacks DB, et al. Guidelines and recommendations for laboratory analysis in the… 2023.7 Umpierrez GE, et al. Hyperglycemic crises in adults with diabetes. 2024.}

Starvation, prolonged vomiting, low-carbohydrate intake, malabsorption, and strenuous exercise can also increase ketone production. Acetoacetate degrades and acetone volatilizes during storage. Free sulfhydryl compounds, some medications, and intense urine pigment can interfere with nitroprusside reactions.

Heme and bile-pigment findings

Blood

The blood pad detects the pseudoperoxidase activity of heme. Intact erythrocytes often create a speckled pattern as individual cells lyse on the pad. Free hemoglobin and myoglobin tend to produce a more uniform color. The pattern suggests the source and does not identify the pigment.

A positive pad with erythrocytes on microscopy supports hematuria. When microscopy shows few or no erythrocytes, a positive pad suggests hemoglobinuria, myoglobinuria, lysed cells, or chemical interference. Menstrual contamination, oxidizing agents, microbial or plant peroxidases, ascorbic acid, high nitrite, high specific gravity, and an unmixed specimen can alter susceptible methods. A positive blood pad calls for microscopic examination to confirm microhematuria and count erythrocytes. ^{1,81 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.8 Barocas DA, et al. Updates to microhematuria. 2025.}

Clinical findings and additional tests distinguish the heme source. Creatine kinase and evidence of muscle injury support myoglobinuria. Plasma hemolysis markers and plasma color support hemoglobinuria. The historical ammonium sulfate precipitation test is method-dependent and has limited current use.

Bilirubin and urobilinogen

Conjugated bilirubin is water soluble and can enter urine. Albumin-bound unconjugated bilirubin is not filtered in an appreciable amount. A urine bilirubin pad uses a diazo reaction to form a colored azo compound. Light exposure, delayed testing, ascorbic acid, and high nitrite can lower bilirubin recovery. Phenazopyridine, indican, and other pigments can complicate color interpretation. A validated bilirubin tablet test can help investigate a questionable strip result. It uses a different test format but also relies on diazo chemistry. Review its own sensitivity and interference limits.^{1010 Siemens Healthineers. Ictotest Reagent Tablets.}

Intestinal bacteria convert conjugated bilirubin to urobilinogen compounds. Some urobilinogen is reabsorbed, and a small portion appears in urine. Multistix uses Ehrlich chemistry based on p-diethylaminobenzaldehyde. Other products use a diazonium reaction. Formalin, light, delayed testing, high nitrite, porphobilinogen, medications, and pigment can interfere with specific systems. Strip performance near the low end cannot reliably establish a complete absence of urobilinogen.^{1,21 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.2 Siemens Healthineers. Multistix 10 SG Reagent Strips. 2017.}

Correlating urine bilirubin with urobilinogen produces three classic patterns:

Interpret these urine patterns with serum bilirubin fractions, liver enzymes, hemolysis studies, and clinical findings.

Nitrite and leukocyte esterase findings

The nitrite pad uses the Griess reaction. Many urinary gram-negative organisms reduce dietary nitrate to nitrite, which forms a pink azo dye on the pad. A positive result is relatively specific for bacteriuria in an appropriately collected specimen. Sensitivity varies with organism, dietary nitrate, bladder incubation time, bacterial concentration, antibiotic exposure, urine concentration, and ascorbic acid. Gram-positive organisms, yeasts, and organisms with limited nitrate-reducing activity may produce a negative result. A negative nitrite result cannot exclude a urinary tract infection.^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

The leukocyte esterase pad detects esterase activity from granulocytes and some monocytes. An esterase hydrolyzes a substrate, and the product couples with a diazonium salt and forms a purple color. Lysed leukocytes can produce a positive reaction even when fewer intact cells remain for microscopy. Lymphocytes contribute little to the conventional reaction. High protein, glucose, specific gravity, oxalate, ascorbic acid, some medications, and early reading can reduce results in susceptible systems. Oxidants, formalin, pigment, and selected drugs can increase or obscure color. Leukocyte esterase color often develops later than other pad reactions, so read the pad at the product's stated time.

Interference flags and follow-up testing

Some strip systems include an ascorbic acid pad. A positive result indicates that vitamin C may suppress susceptible blood, glucose, bilirubin, nitrite, or leukocyte esterase reactions. Other products use chemical layers or reagents intended to reduce particular ascorbate effects. The product insert specifies which reactions need caution after a positive flag, because vitamin C does not invalidate every pad. The Multistix insert reports no interference from ascorbic acid at concentrations normally found in urine. That product claim does not establish resistance at every concentration or for other strip systems.^{22 Siemens Healthineers. Multistix 10 SG Reagent Strips. 2017.}

Pigment, turbidity, medications, preservatives, cleaners, specimen age, and extreme concentration can also produce discordant results. Visual interpretation is especially uncertain when the specimen color resembles an endpoint color. Automated readers standardize illumination and timing and do not remove chemical interference.

Choose follow-up testing for the suspected interference. A different test format may share reaction chemistry, so check whether it supplies an independent measurement:

ASCP BOC examination values

The American Society for Clinical Pathology (ASCP) Board of Certification (BOC) content guideline lists the following urine values for examination preparation. These examination values do not replace a laboratory's validated clinical reference intervals.^{99 American Society for Clinical Pathology… MLS(ASCP) and MLS(ASCPi) Examination Content Guideline. 2026.}

Practice