Introduction
- Uric acid is the final end product of purine metabolism in the human body.
- It is formed mainly from breakdown of nucleic acids, nucleoproteins, and purine-containing compounds.
- Purines from food and body cell turnover are converted into uric acid in the liver.
- Uric acid is transported in blood and excreted mainly by kidneys.
- A small amount is also eliminated through intestine.
- Serum uric acid estimation is an important biochemical test used to assess purine metabolism and renal excretory function.
- Abnormal uric acid levels may indicate metabolic disorders, renal disease, gout, hematological disorders, and endocrine abnormalities.
- Increased serum uric acid is called hyperuricemia.
- Decreased serum uric acid is called hypouricemia.
Principle
- Uric acid estimation is based on enzymatic uricase-peroxidase method.
- The reaction occurs in two enzymatic steps.
First Reaction
- Uric acid is oxidized by uricase enzyme.
- Allantoin, carbon dioxide, and hydrogen peroxide are formed.
Reaction
Uric Acid + O₂ + H₂O → Allantoin + CO₂ + H₂O₂
Second Reaction
- Hydrogen peroxide reacts with 4-aminoantipyrine (4-AAP) and DHBS in presence of peroxidase.
- A colored quinoneimine dye is formed.
Reaction
DHBS + 4-AAP + H₂O₂ → Quinoneimine dye + H₂O
Principle of Measurement
- Intensity of colored dye is directly proportional to uric acid concentration.
- Absorbance is measured at 505 nm.
Specimen
Sample Type
- Serum is preferred specimen
- Plasma can also be used
- Heparin plasma acceptable
- EDTA plasma acceptable
- Urine can also be used
Precautions
- Use non-hemolyzed sample
- Avoid contaminated specimen
- Fresh sample preferred
Stability
Serum / Plasma
- 3 days at 20–25°C
- 7 days at 4–8°C
- 6 months at −20°C
Urine
- 4 days at 20–25°C
Urine Collection
- Use 24-hour urine sample
- Add 15 ml of 5 mol/L NaOH to urine container
- Maintain urine pH above 8
- Dilute urine sample 1:9 before analysis
- Multiply result by 10
Reagents
Reagent 1
- Pipes buffer (pH 7.0)
- DHBS
- Uricase
- Peroxidase
- 4-aminoantipyrine
Standard Reagent
- Uric acid standard solution
Role of Reagents
- Uricase – Oxidizes uric acid
- Peroxidase –Catalyzes color reaction
- 4-AAP and DHBS – Produce colored quinoneimine dye
Materials Required
- Test tubes
- Micropipette
- Pipette tips
- Colorimeter / semi-auto analyzer
- Cuvette
- Timer
- Distilled water
- Uric acid reagent kit
Procedure
| Components | Blank | Standard | Test |
|---|---|---|---|
| Reagent 1 | 1.00 ml | 1.00 ml | 1.00 ml |
| Distilled water | 0.025 ml | — | — |
| Standard | — | 0.025 ml | — |
| Sample | — | — | 0.025 ml |
Incubation
- Mix all tubes properly
- Incubate at 37°C for 5 minutes
Reading
- Measure absorbance of test and standard against blank
- Read at 505 / 670 nm
Calculation
Formula
Uric Acid (mg/dL) = Absorbance of Test / Absorbance of Standard × Standard concentration
Example
- Test absorbance = 0.45
- Standard absorbance = 0.50
- Standard concentration = 6 mg/dL
Calculation
- 0.45 / 0.50 × 6 = 5.4 mg/dL
Normal Reference Values
Serum
| Group | Normal Value |
|---|---|
| Male | 3.5 – 7.2 mg/dL |
| Female | 2.6 – 6.0 mg/dL |
Urine (24 Hours)
| Diet Type | Value |
|---|---|
| Average diet | 250 – 750 mg/day |
| High purine diet | < 1000 mg/day |
| Low purine diet | < 480 mg/day |
Clinical Significance
- Uric acid is the final metabolic product of purine degradation in humans.
- It is formed mainly in the liver from breakdown of nucleic acids, nucleoproteins, and purine bases such as adenine and guanine.
- Normally uric acid is filtered by the kidneys and excreted in urine.
- Therefore serum uric acid level reflects both purine metabolism and renal excretory function.
- Abnormal serum uric acid concentration is clinically important because it is associated with many metabolic, renal, hematological, endocrine, and cardiovascular disorders.
Clinical Importance of Increased Uric Acid (Hyperuricemia)
- Hyperuricemia means increased serum uric acid above normal reference range.
- It occurs either because of increased production of uric acid or reduced renal excretion.
- Persistent hyperuricemia may lead to deposition of monosodium urate crystals in tissues.
Gout
- Gout is the most important disease associated with hyperuricemia.
- In gout, excess uric acid forms monosodium urate crystals.
- These crystals deposit in joints and surrounding tissues.
- This causes acute inflammatory arthritis.
Common Clinical Features
- severe joint pain
- redness
- swelling
- tenderness
- commonly affects great toe
Chronic Gout May Cause
- tophi formation
- joint deformity
- chronic arthritis
Laboratory Importance
- Raised serum uric acid strongly supports diagnosis of gout
- Uric acid monitoring is important during treatment
Renal Disease
- Kidney is the major organ responsible for uric acid excretion.
- Reduced renal function causes uric acid retention.
Hyperuricemia Seen In
- chronic kidney disease
- renal failure
- nephritis
- decreased glomerular filtration
Clinical Importance
- Persistent high uric acid may further damage kidneys
- Uric acid crystals may form renal stones
Uric Acid Stones (Urolithiasis)
- Excess uric acid may precipitate in urinary tract.
- This leads to uric acid stone formation.
Seen In
- acidic urine
- dehydration
- chronic hyperuricemia
Clinical Features
- renal colic
- flank pain
- hematuria
Hematological Disorders
- Rapid destruction of nucleated cells increases purine breakdown.
- This causes increased uric acid production.
Seen In
- leukemia
- polycythemia
- lymphoma
- hemolytic disorders
During Chemotherapy
- Massive tumor cell destruction causes sudden uric acid rise
- This is called tumor lysis syndrome
Metabolic Disorders
Diabetes Mellitus
- Hyperuricemia may occur in diabetes because of altered renal handling and metabolic disturbance.
Obesity
- Increased body mass increases uric acid production.
- Obesity commonly associates with hyperuricemia.
Metabolic Syndrome
- Uric acid often rises in metabolic syndrome.
- It correlates with insulin resistance.
Endocrine Disorders
Hypothyroidism
- Reduced metabolism decreases uric acid clearance.
- Serum uric acid becomes elevated.
Hyperparathyroidism
- May also produce hyperuricemia.
Cardiovascular Significance
- Elevated uric acid is increasingly recognized as cardiovascular risk marker.
- High uric acid is associated with endothelial dysfunction.
Seen In
- hypertension
- atherosclerosis
- coronary artery disease
Clinical Importance
- Persistent hyperuricemia may indicate increased cardiovascular risk.
Increased Uric Acid in Dietary Causes
- High purine diet increases uric acid production.
Foods Causing Increase
- red meat
- organ meat
- seafood
- alcohol
Clinical Importance
- Dietary restriction is important in gout management.
Drug-Induced Hyperuricemia
Drugs Causing Increased Uric Acid
- diuretics
- low dose aspirin
- pyrazinamide
- ethambutol
Clinical Importance of Decreased Uric Acid (Hypouricemia)
- Hypouricemia means decreased serum uric acid below normal level.
- It is less common than hyperuricemia.
Seen In
Wilson Disease
- Copper metabolism disorder causes low uric acid level.
Severe Liver Disease
- Reduced purine metabolism lowers uric acid formation.
Renal Tubular Defects
- Increased uric acid loss through kidneys.
Diagnostic Importance of Uric Acid Test
- Diagnosis of gout
- Monitoring gout treatment
- Assessing renal function
- Detecting purine metabolism disorders
- Monitoring chemotherapy complications
- Evaluating renal stone risk
Monitoring Importance
- Follow-up of gout patients
- Monitoring uric acid lowering therapy
- Assessing dietary control
- Monitoring chronic kidney disease
