Understanding uric acid testing and its role in health diagnostics is crucial for lab professionals. This guide provides a step-by-step guide on running a uric acid test on chemistry analyzers, including sample preparation, reagent handling, and instrument programming. In addition to the technical details, we'll explore uric acid metabolism, its clinical significance, and the ideal times to conduct uric acid tests.
At Labex International, our goal is to provide reliable diagnostic tools that empower laboratories to deliver accurate and actionable results.
What is Uric Acid and Why is it Important?
Uric acid is a waste product formed from the natural breakdown of purines, which are found in certain foods and cells. The majority of uric acid is dissolved in the blood, processed by the kidneys, and excreted through urine. Elevated uric acid levels, known as hyperuricemia, can lead to conditions like gout—a type of arthritis caused by uric acid crystal deposits in joints—and may indicate kidney dysfunction.
Uric Acid Metabolism
- Purine metabolism: Uric acid is the final oxidation product of purines, such as adenine and guanine.
- Production: It is primarily produced in the liver but is also synthesized in other tissues.
- Excretion: Normally, two-thirds of uric acid is excreted via the kidneys and the remainder through the intestines. When this balance is disrupted, uric acid accumulates in the blood, leading to hyperuricemia.
Why Test for Uric Acid?
Uric acid testing is critical for diagnosing and managing several medical conditions:
- Gout: Characterized by sudden, severe joint pain, typically in the big toe, gout is often triggered by hyperuricemia.
- Kidney Stones: High levels of uric acid can lead to the formation of kidney stones.
- Renal Impairment: Poor kidney function often leads to uric acid retention.
- Other Conditions: Some metabolic and cardiovascular conditions, including hypertension and heart disease, are associated with elevated uric acid levels.
When to Conduct a Uric Acid Test
A healthcare provider may order a uric acid test if a patient presents with:
- Symptoms of gout: Joint pain, swelling, and redness.
- Kidney stones: Symptoms include severe pain in the side or lower back.
- Chronic kidney disease: Unexplained elevations in blood urea nitrogen (BUN) or creatinine levels.
- Metabolic concerns: Hyperuricemia is also associated with certain metabolic conditions like diabetes and obesity.
Tests may be conducted as part of routine health screenings or when there are signs of renal dysfunction or gout flare-ups.
Sample Preparation for Uric Acid Testing
Sample Types:
- Serum or Plasma: Preferred for routine uric acid testing. Samples can be stored for 3-5 days at 2-8°C or up to 6 months at -20°C.
- Urine (24-hour collection): Often used when evaluating for kidney stones or assessing the body’s ability to eliminate uric acid. Stability lasts up to 4 days at 15-25°C.
Sample Handling Tips:
- For urine samples, maintain the pH above 8 by using preservatives or warming cloudy samples to 60°C to dissolve precipitated urates.
- Use clean, non-contaminated containers for both serum and urine samples to avoid interference.
Reagents for Uric Acid Testing
Labex International provides complete solutions for chemistry kit to quantitative determination of uric acid, utilizing an enzymatic method.
Reagents:
- R1 (Buffer): 50 mmol/L phosphate buffer (pH 7.4) and 4 mmol/L DCPS (2-4 Dichlorophenol sulfonate).
- R2 (Enzymes): Contains 60 U/L uricase, 660 U/L peroxidase (POD), 200 U/L ascorbate oxidase, and 1 mmol/L 4-aminophenazone (4-AP).
- Uric Acid Standard: Aqueous primary standard (6 mg/dL).
Principle of the Test
Uric acid is oxidized by uricase to allantoin and hydrogen peroxide (H₂O₂). The hydrogen peroxide reacts with peroxidase (POD), 4-aminophenazone, and DCPS to form a red quinoneimine compound. The intensity of the red color is directly proportional to the uric acid concentration, and this can be measured photometrically at 520 nm.
Reagent Preparation
To prepare the working reagent (WR):
- Mix equal volumes of R1 Buffer and R2 Enzymes.
- The working reagent is stable for one week at 2-8°C or four days at room temperature (15-25°C).
Setting Up and Programming the Chemistry Analyzer
Labex International's reagent kit can be used with most automatic chemistry analyzers. Here’s how to set up your equipment for accurate testing:
1. Instrument settings:
- Method : End-point
- Wavelength/Filter : Set at 546 nm (or within the range of 490-550 nm).
- Secondary Wavelength : Not Required
- Sample Volume : 450 uL
- Unit : mg/dL
- Delay/Lag Time : 5 seconds
- Measuring time : 5 seconds
- Factor : 1.0 (initially, This will be calculated by the analyzer after calibration)
- Reagent Blank : Optional
- Sample Blank : Not required
- Standard : No. of standards: 1, Concentration :6.0, Rep. 1
- Cuvette : Use a 1 cm light path cuvette.
- Temperature : Ensure the analyzer is set to 37°C or between 15-25°C.
- Water Blank : Run the distilled water to zero the instrument using distilled water.
2. Pipetting:
Take 3 test tubes and mark them B for Blank, S for standard and Lab no. ( Patient details) for sample tubes and pipette the below reagents to the relevant tubes.
- For the Blank : Add 500 uL of the working reagent (WR).
- For the Standard : Add 500 uL WR and 10 µL of the uric acid standard.
- For the Sample : Add 500 mL WR and 10 µL of the patient sample (serum, plasma, or urine).
3. Mix and Incubate:
- Mix thoroughly and incubate for 5 minutes at 37°C or 10 minutes at room temperature.
- Read the water blank after selecting the programe to zero the instrument.
- Sip/Read reagent blank to minus the color concentration of reagent
- Sip/Read Standard to calibrate the instrument. Make sure to check the graph (should be linear) and factor calculated after reading. Factor can varry from instrument to instrument and kit to kit. it can also vary according to the climate, pipetting methods, and many more.
- Sip/Read Samples to note sample concentration (Patient Value/Result)
Calculating the Uric Acid Concentration
Automated and semi automated chemistry analzyers read the absorbance of the reagent and calculate the concentration according to the calibration curve. You don't have to calculate manually, however if you have to here the formula:
For serum/plasma:
Uric Acid (mg/dL) = (A_{Sample} - A_{Blank}) / (A_{Standard} - A_{Blank}) × 6
For urine:
Uric Acid (mg/24h) = (A_{Sample} - A_{Blank}) / (A_{Standard} - A_{Blank}) × 6 × Volume (dL)
Quality Control and Calibration
Quality control is essential to ensure the reliability of your uric acid test results.
- Use CONTROL NORMAL and CONTROL PATHOLOGICAL to monitor test performance.
- CALIBRATOR or Standard is used to calibrate the analyzer.
- Establish a regular quality control schedule and take corrective action if control values fall outside the acceptable range.
When Should You Re-Run a Test?
If control values or test results fall outside the expected ranges, consider the following steps:
- Check the analyzer settings.
- Re-calibrate the equipment.
- Verify the quality of reagents (check for expiration or contamination).
Conclusion
Running a uric acid test using chemistry analyzers is a fundamental procedure for diagnosing gout, kidney disease, and other health conditions. At Labex International, we are committed to providing high-quality reagents and reliable diagnostic solutions that empower healthcare professionals to deliver accurate results.
Partner with Labex International for all your laboratory needs and ensure precision in every test you conduct. For more details on our products and services, visit our website or contact us directly.
By delivering accurate, reliable, and timely uric acid testing, your laboratory can play a critical role in patient care.
Step by step guide on Uric Acid test, Method and Techniques