Kidney Stones

Kidney stone formation is predisposed by a number of factors, including dehydration, a high- protein diet, and a sedentary lifestyle. Increased levels of calcium, urate, and oxalate and decreased levels of magnesium may promote stone formation. The solubility of the urine chemicals may be influenced by changes in urine pH. Calcium oxalate is the most common stone found. Stones are associated with conditions in which the urine is highly concentrated.

Uric acid is produced from the cellular breakdown products of the purine nucleosides, adenosine and guanosine. Uric acid is one of several compounds that are sometimes called nonprotein nitrogen (NPN) compounds. Most uric acid is produced from the natural cellular breakdown in the body; some uric acid is produced from the breakdown of dietary purines. Individuals who are producing abnormal levels of cells, such as in a leukemia process, may have high levels of uric acid. Uric acid is filtered through the renal glomerulus and is almost completely reabsorbed in the proximal and distal tubules. At pH levels above 5.57, most uric acid is in the urate form, a chemical form that is soluble in urine. At pH levels below 5.57, levels of uric acid formation increase.

Hyperuricemia is a term used to describe individuals who have increased uric acid in the blood. Such individuals may be asymptomatic, but are at increased risk for associated renal problems. Gout is a disease in which urates are deposited in body fluids. The classic symptoms of gout include gouty arthritis, in which uric acid is deposited in joints. Gout may be due to overproduction of purines or to defects in the metabolic pathways of purine metabolism. Methods of measuring uric acid are presented in Test Methodology 6-8.

nephrolithiasis - presence of calculi in the urinary tract; urate nephrolithiasis indicates the presence of uric acid in the stone calculi - any abnormal concretion of precipitated organic materials, commonly called a stone, within the body (singular: calculus)

4 Common Sense Check

On refrigeration of urine specimens, crystals often form around small preformed crystals or cellular elements in the urine. Stone formation is unusual because large crystal calculi form at body temperature.

Uric Acid Lipid Metabolism

TEST METHODOLOGY 6-8. URIC ACID (continued)

1. Decrease in absorbance at 293 nm is due to loss of uric acid.

2. Alternate methods have hydrogen peroxide converted to a chromagen using 4-aminophenazone and peroxidase in a second enzyme-catalyzed reaction.

3. Excessive amounts of protein and lipids can interfere with results, and negative interferences often occur with the presence of xanthine and hemoglobin.

Phosphotungstate Method

This method requires a protein-free filtrate for serum or plasma.

Uric acid + phosphotungstic acid + Na2CO2 alkaline pH ) allantoin + CO2 + tungsten blue

Proteins in normal amounts in serum must be removed to prevent interferences. Glucose, ascorbic acid, glutathione, hemoglobin, and drugs such as acetaminophen and caffeine commonly interfere.

The Specimen

Serum, heparinized plasma, and urine may be tested. Anticoagulants containing fluoride or citrate should not be used. Hemolysis and lipemia should be avoided.

Reference Ranges

Uncase Method

Female 2.6-6.0 mg/dL

Phosphotungstate Method Male 4.2-8.0 mg/dL

Female 3.5-7.3 mg/dL

Values are lower in children.

Case Scenario 6-9 Renal Calculi: Kidney Stones Follow-Up:

The patient was treated with ultrasound lithotripsy, in which sound waves crush the stones inside the body. The patient walked out of the emergency upright, but may expect to experience reoccurrences of the event.

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