Principles of Uric Acid Quantification Methods, Sample Analysis, and Applications in Disease Research
Principles of Uric Acid Quantification Methods, Sample Analysis, and Applications in Disease Research
Uric acid is the major end product of purine catabolism in humans and exists primarily as free uric acid or urate in blood, urine, and tissues. Uric acid quantification is a commonly used technique in clinical biochemical analysis and metabolic research and is widely applied in hyperuricemia screening, gout research, evaluation of renal urate excretion, analysis of uric acid stones, and studies of urate-lowering drugs.
Keywords: uric acid; uricase; colorimetric assay; high-performance liquid chromatography; serum; urine; hyperuricemia; gout; kidney disease
1 Uric Acid Production, Metabolism, and Excretion
1.1 Uric Acid Production
Purines derived from cellular nucleic acid turnover and dietary sources are degraded to hypoxanthine and xanthine, which are subsequently converted into uric acid by xanthine oxidoreductase:
Hypoxanthine → Xanthine → Uric acid
Xanthine oxidoreductase can exhibit either xanthine dehydrogenase or xanthine oxidase activity. Increased purine-substrate availability, enhanced nucleic acid degradation, or increased xanthine oxidase activity can all promote uric acid production.
1.2 Forms of Uric Acid
Uric acid is a weak acid and exists mainly in the form of urate in blood and interstitial fluid. When blood uric acid remains elevated, urate in body fluids may become supersaturated and form monosodium urate crystals. When urinary pH decreases, the degree of uric acid dissociation and its solubility decline, making crystallization more likely. Therefore, uric acid stone formation is associated not only with urinary uric acid content but also with urinary pH and urine volume.
1.3 Uric Acid Excretion
Uric acid is eliminated mainly through the kidneys and intestine. After filtration through the glomeruli, uric acid undergoes reabsorption, secretion, and re-reabsorption in the proximal renal tubules. Transporters including URAT1, GLUT9, members of the OAT family, and ABCG2 jointly participate in transmembrane urate transport. Increased uric acid production, decreased glomerular filtration, enhanced tubular reabsorption, or reduced intestinal excretion can all increase blood uric acid. Hyperuricemia in different individuals may result from overproduction, underexcretion, or a combination of both mechanisms.
2 Clinical and Research Significance of Uric Acid Quantification
2.1 Blood Uric Acid Measurement
Serum or plasma uric acid reflects the combined effects of uric acid production, distribution in body fluids, and excretion. It can be used for hyperuricemia screening, follow-up of patients with gout, evaluation of urate-lowering drugs, and metabolic monitoring in patients with impaired renal function. Elevated blood uric acid is an important prerequisite for urate crystal formation, but blood uric acid results alone cannot establish a diagnosis of gout. Clinical symptoms, synovial-fluid crystal analysis, and imaging findings should also be considered.
2.2 Urinary Uric Acid Measurement
Urinary uric acid measurement reflects renal urate excretion. Twenty-four-hour urinary uric acid is suitable for studying urate-excretion phenotypes, evaluating uric acid stone risk, and investigating uricosuric drugs. A urinary uric acid-to-creatinine ratio can be measured in spot urine samples to correct for urine dilution, but it cannot fully replace standardized 24-hour urine collection.
2.3 Research Applications
Uric acid measurement can be used to evaluate hyperuricemia animal models, screen xanthine oxidase inhibitors, investigate urate transporters, and analyze uric acid-associated inflammation, oxidative stress, and renal injury. Research studies generally also measure hypoxanthine, xanthine, creatinine, blood urea nitrogen, inflammatory cytokines, and renal histopathological changes to avoid interpreting complex pathological processes from a single uric acid indicator.
3 Uric Acid Quantification Methods and Reaction Principles
3.1 Phosphotungstic Acid Reduction Method
(1) Detection Principle
Under alkaline conditions, uric acid reduces phosphotungstic acid to generate blue tungsten oxides. Within a defined range, the absorbance of the colored product is associated with uric acid content, and the uric acid concentration in a sample can be calculated from a standard curve.
(2) Advantages
This method has a simple reaction principle and relatively low instrumentation requirements. Measurement can be completed using a conventional visible-light spectrophotometer, and reagent costs are relatively low.
(3) Limitations
Phosphotungstic acid can also be reduced by ascorbic acid, glutathione, cysteine, and other reducing substances, producing falsely elevated results. Incomplete protein precipitation, sample color, and turbidity can also interfere with absorbance measurements.
(4) Applicable Samples
This method is mainly suitable for uric acid standard solutions, artificial simulated samples with defined compositions, optimization of color-development conditions, and teaching experiments. It can also be used to reproduce traditional chemical methods for uric acid measurement. When used with serum or plasma, deproteinization should be performed, and results should be validated using sample blanks, spike-recovery experiments, and method-comparison studies. Tissue homogenates and cell lysates contain large amounts of proteins, pigments, and reducing substances and are not suitable for direct and accurate measurement using this method.
3.2 Uricase Ultraviolet Method
(1) Detection Principle
Uric acid has characteristic ultraviolet absorption at approximately 292-293 nm. After uricase is added, urate is first oxidized to 5-hydroxyisourate, with simultaneous formation of hydrogen peroxide. The 5-hydroxyisourate subsequently undergoes spontaneous decomposition or downstream enzyme-catalyzed reactions to form allantoin and release carbon dioxide. The decrease in ultraviolet absorbance before and after the reaction is associated with uric acid content:
Urate + O₂ + H₂O —Uricase→ 5-Hydroxyisourate + H₂O₂
5-Hydroxyisourate undergoes subsequent hydrolysis and decarboxylation → Allantoin + CO₂
(2) Advantages
Uricase has relatively high substrate specificity for uric acid. The assay does not require a coupled chromogenic substrate and can reduce interference from nonspecific chemical reduction reactions.
(3) Limitations
Nucleic acids, proteins, and other ultraviolet-absorbing substances can increase the detection background, while sample turbidity may cause light scattering. The sensitivity is generally lower than that of fluorescence assays and certain coupled colorimetric methods.
(4) Applicable Samples
This method is suitable for clear serum, validated plasma samples, appropriately diluted urine, and purified reaction systems. It can also be used to investigate uricase catalytic activity and uric acid consumption. Tissue homogenates, cell lysates, and high-protein samples should first undergo centrifugation, deproteinization, or ultrafiltration, and a sample blank without uricase should be included.
3.3 Uricase-Peroxidase Coupled Colorimetric Method
(1) Detection Principle
Urate is first converted by uricase into 5-hydroxyisourate and hydrogen peroxide. Peroxidase then catalyzes the oxidative coupling of hydrogen peroxide with a chromogenic substrate to form a colored product. The 5-hydroxyisourate can subsequently be converted into allantoin with the release of carbon dioxide:
Urate + O₂ + H₂O —Uricase→ 5-Hydroxyisourate + H₂O₂
H₂O₂ + Chromogenic substrate —Peroxidase→ Colored oxidation product
Within the linear range of the reaction, the absorbance of the colored product is directly proportional to the uric acid content.
(2) Advantages
This method has relatively high specificity and sensitivity, uses mild reaction conditions, and can be readily adapted to automated biochemical analyzers, spectrophotometers, or microplate readers for batch testing. It is one of the most commonly used methods for measuring uric acid in blood and urine.
(3) Limitations
Ascorbic acid, glutathione, and other reducing substances can consume hydrogen peroxide or reduce the colored product, producing falsely low results. Bilirubin, hemoglobin, lipid particles, and colored drugs may also cause optical interference. Changes in the activity or stability of uricase, peroxidase, or the chromogenic substrate can affect color intensity and test results.
(4) Applicable Samples
This method is suitable for serum, validated plasma samples, and appropriately diluted urine and is generally preferred for routine uric acid measurement and large-scale sample analysis. When used with tissue homogenates, cell lysates, samples treated with colored drugs, or plant extracts, sample blanks, spike-recovery tests, and dilution-parallelism tests should be performed. If color-related or reducing-substance interference cannot be eliminated, a chromatographic method is more appropriate.
3.4 Fluorescence Method
(1) Detection Principle
Uric acid is oxidized by uricase to generate hydrogen peroxide. In the presence of peroxidase, hydrogen peroxide oxidizes a fluorescent probe to form a fluorescent product with characteristic excitation and emission wavelengths. Fluorescence intensity is associated with uric acid content.
(2) Advantages
Fluorescence assays have relatively high sensitivity, require only small sample volumes, and are suitable for microplate analysis and low-level uric acid measurement.
(3) Limitations
Tissue pigments, culture-medium components, and drugs may exhibit autofluorescence or cause fluorescence quenching. Antioxidants, peroxidase inhibitors, and light-exposure conditions may also affect the detection signal.
(4) Applicable Samples
This method is suitable for small-volume serum samples, diluted urine, cell-culture supernatants, and pretreated tissue homogenates and cell lysates. It is particularly suitable for small-volume samples and high-throughput experiments. For complex samples, blanks without fluorescent probe, blanks without uricase, and matrix-spiked controls should be included to exclude autofluorescence and reaction inhibition.
3.5 High-Performance Liquid Chromatography
(1) Detection Principle
High-performance liquid chromatography separates uric acid from other sample components according to differences in their distribution between the stationary and mobile phases. Quantification is then performed using ultraviolet, diode-array, or electrochemical detection.
(2) Advantages
Chromatographic separation reduces interference from proteins, pigments, drugs, and other reducing substances and enables simultaneous measurement of purine metabolites such as hypoxanthine, xanthine, and uric acid.
(3) Limitations
This method requires chromatographic instrumentation, columns, and standardized sample preparation. Analysis time and operating costs are higher than those of routine enzymatic methods. Column condition, mobile-phase composition, and retention-time drift may all affect quantitative results.
(4) Applicable Samples
High-performance liquid chromatography is suitable for serum, plasma, urine, tissue homogenates, cell lysates, and complex samples containing drugs or colored components. It is particularly useful for drug-interference studies, purine-metabolism pathway analysis, and confirmation of abnormal uricase-method results. Blood and tissue samples generally require protein precipitation, ultrafiltration, or solid-phase extraction.
3.6 Liquid Chromatography-Tandem Mass Spectrometry
(1) Detection Principle
Liquid chromatography-tandem mass spectrometry first separates uric acid chromatographically and then performs qualitative and quantitative analysis according to its mass-to-charge ratio and characteristic fragment ions. Stable isotope-labeled uric acid can be used as an internal standard to correct for sample-preparation losses and matrix effects.
(2) Advantages
This method has high specificity, sensitivity, and quantitative accuracy and enables combined analysis of uric acid and related metabolites in complex matrices.
(3) Limitations
Mass spectrometers, isotope-labeled internal standards, and instrument maintenance are costly. The method also requires advanced sample-preparation, instrument-operation, and data-analysis capabilities. Matrix-induced ion suppression may affect test results.
(4) Applicable Samples
This method is suitable for establishing reference procedures, assigning values to reference materials, metabolomics studies, analysis of complex tissue samples, and simultaneous quantification of multiple purine metabolites. It is not the preferred method for routine high-throughput uric acid screening in ordinary laboratories.
4 Performance Comparison of Chemical, Enzymatic, and Instrumental Methods
Table 1 Performance Comparison of Uric Acid Quantification Methods
Detection Method | Detection Specificity | Detection Sensitivity | Major Advantages | Major Limitations | Major Applicable Samples |
Phosphotungstic acid reduction method | Relatively low | Moderate | Simple operation and low instrumentation requirements | Susceptible to interference from reducing substances | Standard solutions, artificial simulated samples, and teaching experiments |
Uricase ultraviolet method | Relatively high | Moderate | Simple reaction system without chromogenic substrates | Susceptible to ultraviolet background and turbidity | Clear serum, plasma, diluted urine, and purified systems |
Uricase coupled colorimetric method | Relatively high | Relatively high | Highly automated and suitable for batch testing | Affected by ascorbic acid, bilirubin, and sample color | Serum, plasma, and urine |
Fluorescence method | Relatively high | High | Requires little sample and is suitable for high-throughput testing | Affected by autofluorescence and fluorescence quenching | Small-volume blood samples, culture supernatants, and pretreated tissue samples |
HPLC | High | Relatively high | Separates interfering substances and enables combined metabolite analysis | Requires relatively lengthy sample preparation and analysis | Blood, urine, tissues, and drug-treated samples |
LC-MS/MS | Very high | Very high | High specificity and accuracy and supports isotope-labeled internal standards | High instrumentation and technical costs | Reference analysis, metabolomics, and complex biological samples |
5 Detection Characteristics of Serum, Plasma, Urine, and Tissue Samples
5.1 Serum Samples
Serum is one of the most commonly used sample types for uric acid measurement. After blood collection, the blood should be allowed to clot completely, and the serum should be separated promptly by centrifugation to reduce the effects of continued blood-cell metabolism and prolonged contact with cells. Severe hemolysis, icterus, and lipemia may interfere with colorimetric or ultraviolet detection. For long-term storage, serum should be aliquoted and stored at low temperature to avoid repeated freeze-thaw cycles.
5.2 Plasma Samples
Heparinized plasma can be used in certain uric acid assay systems, but the anticoagulant types permitted by the method should be confirmed before use. EDTA and citrate may alter the ionic composition of the sample or cause dilution effects and should not be used directly without validation. The same sample type should be used throughout a study, and results from serum and plasma prepared using different anticoagulants should not be directly combined.
5.3 Urine Samples
Urinary uric acid concentration is strongly influenced by water intake, urine volume, collection period, diet, and urinary pH. During 24-hour urine collection, total urine volume should be recorded, and the sample should be thoroughly mixed before aliquoting. The measured concentration should then be converted into total 24-hour excretion.
Uric acid can crystallize in acidic urine, causing falsely low results when only the supernatant is analyzed. Samples should be inspected for precipitates or crystals before testing and should be mixed, dissolved, adjusted for pH, or diluted according to the assay method. Urinary uric acid concentrations are generally higher than blood concentrations, so enzymatic assays should confirm that diluted results fall within the linear range of the method.
5.4 Tissue Samples
Liver, kidney, joint, and other experimental tissues should be rapidly cooled or frozen after collection to reduce post-sampling metabolic changes. Tissue may be homogenized with a defined volume of precooled buffer according to tissue mass, followed by centrifugation to remove debris. Results are generally normalized to tissue wet weight, total protein, or DNA content.
Tissue homogenates contain proteins, lipids, pigments, and antioxidants. Sample blanks should therefore be included when colorimetric or fluorescence methods are used. If matrix interference is substantial, deproteinization, ultrafiltration, or chromatographic analysis may be required.
5.5 Cells and Culture Supernatants
Uric acid in culture supernatants can reflect cellular uric acid release, changes in uric acid concentration within the culture system, or residual levels following exogenous uric acid treatment. Cell lysates can be used to analyze intracellular uric acid. A cell-free culture-medium blank should be included to subtract the uric acid background contributed by serum, culture medium, and additives. Intracellular results can be normalized to cell number, total protein, or DNA content.
6 Interfering Factors and Quality Control in Uric Acid Measurement
6.1 Physiological and Sampling Factors
Blood uric acid can be affected by a high-purine diet, alcohol consumption, fasting, vigorous exercise, hydration status, sampling time, and medication use. Urinary uric acid is additionally influenced by the completeness of urine collection, urine volume, and urinary pH. Longitudinal studies should standardize dietary conditions, sampling time, dosing time, and sample-processing procedures as much as possible.
6.2 Chemical Interference
Ascorbic acid, glutathione, cysteine, and other reducing substances can affect both the phosphotungstic acid method and peroxidase-coupled colorimetric methods. In the phosphotungstic acid method, they may produce nonspecific color development and falsely elevated results. In the peroxidase-coupled method, they may consume hydrogen peroxide and produce falsely low results. Drugs and their metabolites may also inhibit uricase or peroxidase or participate directly in oxidation-reduction reactions.
6.3 Optical Interference
Bilirubin, hemoglobin, lipid particles, tissue pigments, and colored drugs may interfere with ultraviolet, visible-light, or fluorescence measurements. Sample blanks should be included for colored or turbid samples. Dual-wavelength correction, deproteinization, or chromatographic separation may be required.
6.4 Standard Curves and Matrix Effects
The standard curve should cover the expected concentration range of the samples, and sample signals should fall within the linear range of the method. Samples exceeding the measurement range should be diluted and retested rather than quantified by extrapolating beyond the standard curve.
Serum, urine, and tissue homogenates have different matrix compositions. Spike-recovery and dilution-parallelism experiments should therefore be performed for complex samples to confirm that the assay does not exhibit substantial matrix inhibition or nonlinear responses.
6.5 Quality Control
Each analytical batch should include a reagent blank, standards, and quality-control samples at a minimum of two concentration levels. During method establishment, linear range, precision, accuracy, limit of detection, limit of quantification, and sample stability should be evaluated. Method-comparison studies should be performed when reagent lots, calibrators, instruments, or testing methods are changed. Research experiments should also record sample mass or volume, dilution factor, number of freeze-thaw cycles, and normalization method.
7 Applications of Uric Acid Levels in Hyperuricemia, Gout, and Kidney Disease Research
7.1 Hyperuricemia Research
Hyperuricemia studies generally use blood uric acid as the core indicator and combine urinary uric acid, serum creatinine, urinary creatinine, and renal-function indicators to analyze uric acid production and excretion. Fractional excretion of uric acid can be used to evaluate renal urate-excretion capacity:
Fractional excretion of uric acid (%) =Urinary uric acid concentration × Serum creatinine concentration ÷ Blood uric acid concentration ÷ Urinary creatinine concentration × 100%
Elevated blood uric acid together with reduced fractional excretion of uric acid generally suggests insufficient renal urate excretion. Increased urinary uric acid excretion may be associated with increased uric acid production or the effects of uricosuric drugs. The results should still be interpreted in combination with glomerular filtration rate and medication use.
7.2 Gout Research
Gout is caused by monosodium urate crystal deposition and the resulting inflammatory response. Blood uric acid measurement can be used to evaluate the risk of crystal formation and the effects of urate-lowering therapy but does not independently reflect intra-articular crystal burden or inflammatory severity.
Gout studies should also evaluate joint swelling, pain-related behavior, inflammatory-cell infiltration, inflammatory indicators such as IL-1β, and crystal deposition to distinguish uncomplicated hyperuricemia from urate crystal-induced inflammation.
7.3 Kidney Disease Research
Declining renal function can reduce uric acid clearance and increase blood uric acid. Uric acid and urate crystals may also contribute to tubular obstruction, interstitial inflammation, and uric acid stone formation. Kidney disease studies should analyze blood uric acid together with serum creatinine, blood urea nitrogen, estimated glomerular filtration rate, urinary albumin, urinary pH, and renal histopathology rather than interpreting elevated uric acid as the sole cause of renal injury.
7.4 Uric Acid Stone Research
Uric acid stone formation is closely associated with urinary pH, uric acid concentration, and urine volume. Persistently acidic urine reduces uric acid solubility and may increase crystallization risk even when urinary uric acid excretion is not markedly elevated. In addition to 24-hour urinary uric acid, studies of uric acid stones should record urinary pH, urine volume, and crystallization and should distinguish crystals formed in vivo from precipitates generated during sample storage.
7.5 Animal Models and Drug Evaluation
Most commonly used rodents express uricase, which further converts uric acid into allantoin. Their uric acid metabolism therefore differs from that of humans. Hyperuricemia animal models are commonly established by inhibiting uricase, increasing the purine load, or interfering with urate excretion.
Studies of xanthine oxidase inhibitors should simultaneously measure blood uric acid, urinary uric acid, xanthine oxidase activity, hypoxanthine, and xanthine. Studies of urate-transport modulators can additionally evaluate fractional uric acid excretion and changes in the expression of transporters such as URAT1.
8 Products for Uric Acid Detection, Metabolism, and Renal-Function Research
8.1 Uric Acid Standards, Isotope-Labeled Internal Standards, and Assay Kits
Catalog # | Description | Grade & Purity |
Uric acid | ≥98% | |
Uric acid | BioReagent, ≥99% (HPLC) | |
Uric acid | Moligand™, ≥99% | |
Uric acid | Moligand™, ≥99.8% | |
Uric acid-1,3-¹⁵N₂ | ≥98 atom% ¹⁵N, ≥98% | |
Uric acid-C,N | ≥99% | |
Uric acid sodium salt | Na 11-13% | |
Monosodium urate (PBS solution) | 2 wt% | |
Uric Acid (UA) Content Assay Kit (Uricase, Micro Method) | BioReagent | |
Uric Acid (UA) Content Assay Kit (Uricase, Colorimetric Method) | BioReagent |
8.2 Products Related to Uricase and Coupled Color-Development Systems
Catalog # | Description | Grade & Purity |
Uricase (UAO) | Bioactive, recombinant, ActiBioPure™, high performance, EnzymoPure™, ≥80% (SDS-PAGE), expressed in E. coli, 8-12 U/mg enzyme powder | |
Uricase (UAO) from Candida sp. | Bioactive, ActiBioPure™, Native, high performance, EnzymoPure™, ≥4 U/mg enzyme powder | |
Uricase from Candida utilis | EnzymoPure™, ≥2 units/mg dry weight | |
Urinase from bovine liver | Bioactive, BioReagent, ActiBioPure™, EnzymoPure™, >7.2 U/mg powder | |
Uricase (UA) | Bioactive, ActiBioPure™, high performance, EnzymoPure™, ≥90% (SDS-PAGE), ≥20 U/mg powder | |
Peroxidase from horseradish(EIA Grade,Purified) | EnzymoPure™, RZ 2.9, ≥500 units/mg protein | |
Horseradish Peroxidase (HRP) | Bioactive, ActiBioPure™, Native, high performance, EnzymoPure™, from horseradish, ≥250 U/mg enzyme powder, RZ ≥3 | |
Horseradish Peroxidase (HRP) | Bioactive, ActiBioPure™, Native, high performance, EnzymoPure™, ≥300 U/mg enzyme powder, RZ ≥3, from horseradish | |
4-Aminoantipyrine Hydrochloride [for Biochemical Research] | ≥98% (HPLC) (T) |
8.3 Products for Purine Metabolism, Urate Transport, and Urate-Deposition Research
Catalog # | Description | Grade & Purity |
Hypoxanthine | Moligand™, ≥99% | |
Hypoxanthine | BioReagent, Moligand™, for cell culture, powder | |
Hypoxanthine-¹³C2,¹⁵N | ≥95 atom%, ≥90% | |
Xanthine | Moligand™, ≥99.5% (HPLC) | |
Xanthine | UltraBio™, ≥99% | |
Xanthine Oxidase (XOD) | Bioactive, recombinant, ActiBioPure™, high performance, EnzymoPure™, ≥50 U/mg protein | |
Xanthine Oxidase (XOD) Activity Assay Kit (WST-1, Microscale Method) | BioReagent, suitable for analysis, colorimetric assay | |
Xanthine Oxidase Activity Assay Kit (WST-8, Micro Method) | BioReagent | |
Allantoin | ≥98% | |
Allantoin-C,N | ≥99% | |
Mouse Urate Transporter 1(URAT1) ELISA Kit | BioReagent | |
Human Xanthine Dehydrogenase (XDH) ELISA Kit | BioReagent | |
Rat Xanthine Oxidase(XOD) ELISA Kit | BioReagent | |
Rat Xanthine Dehydrogenase (XDH) ELISA Kit | BioReagent | |
Mouse Xanthine Oxidase (XOD) ELISA Kit | BioReagent | |
Monosodium Urate Staining Solution (Gomori Method) | BioReagent, biological stain, for microscopy |
8.4 Creatinine Standards and Assay Kits
Catalog # | Description | Grade & Purity |
Creatinine | ≥99% | |
Creatinine | 10 mM in water | |
Creatinine-(methyl-d₃) | ≥98 atom% D, ≥97% | |
Creatinine-(methyl-¹³C) | ≥99 atom% ¹³C | |
Creatinine (Cr) Content Assay Kit (PA Rate, Micro Method) | BioReagent | |
Creatinine (Cr) Content Assay Kit (SOX, Micro Method) | BioReagent | |
Creatinine (Cr) Content Assay Kit (SOX, Colorimetric Method) | BioReagent |
Uric acid detection methods should be selected according to sample type and research objective. Routine blood and urine samples can be analyzed using uricase-based methods, whereas chromatographic or mass-spectrometric methods are more suitable for complex tissues and multimetabolite analysis. Sample blanks, spike-recovery experiments, and quality-control procedures should be used to ensure reliable results.
For more related articles, please see below:
[1] Animal models of hyperuricemia and uric acid nephropathy