Clinical Chemistry: Non-Protein Nitrogenous Compounds

Uric Acid

Product of catabolism of the purine nucleic acids (i.e. Adenine and Guanine)

Clinical Utility of Uric Acid

Confirm diagnosis and monitoring treatment of gout

Clinical Utility of Uric Acid

Prevent uric acid nephropathy during chemotherapeutic treatment

Clinical Utility of Uric Acid

Assess inherited disorders of purine metabolism

Clinical Utility of Uric Acid

Detect kidney dysfunction

Clinical Utility of Uric Acid

Assist in the diagnosis of renal calculi/stones

Phosphotungstic Acid Method

Performed in an alkaline solution; Principle: oxidation of uric acid to allantoin via uricase

Enzymatic Methods

Similar first step: catalyzed by uricase; More commonly used

Monosodium Urate

Nearly all of uric acid in plasma is present in this form; Relatively insoluble in plasma (pH of ~7)

Uric Acid Saturation

When uric acid concentration > 6.8 mg/dL: Plasma becomes saturated; Urate crystals may form and precipitate in the tissues

Tophi

Deposit of uric acid; Joints are swelling/inflamed due to uric acid crystals between the bones

Gout

May cause gout (gouty arthritis)

Spectrophotometric: Uricase Method

Result: Decrease in absorbance measured at 293 nm

Peroxidase Coupled Enzymatic Method

Color produced is proportional to the quantity of uric acid

Hypouricemia

Secondary to liver disease; Inability to produce uric acid in the liver

Isotope Dilution Mass Spectrometry (IDMS)

Detection of characteristic fragments following ionization; Quantification using isotopically labeled compound

Uric Acid Crystals

Presence of uric acid crystals in the tissue

Bilirubin and Ascorbic Acid

Can destroy peroxidase in commercial preparations

Potassium Ferricyanide

Added to minimize interference in peroxidase coupled enzymatic method

Uric Acid Concentration in Plasma

10% in plasma concentration

Uric Acid Concentration in Urine

1.7% in urine concentration

Uric Acid Reabsorption

Most uric acid is reabsorbed in the proximal tubules and reused

Specimen Requirements

Serum, heparinized plasma or urine; fasting is not required; avoid gross lipemia; uric acid is stable in plasma or serum after RBCs have been removed; serum may be refrigerated for 3 to 5 days; EDTA (lavender) and fluoride (gray) additives should not be used for uricase methods; collected urine must be alkaline (pH 8).

Reference Values

Conversion factor (conventional to SI) → 0.0595 [mmol/L].

Plasma or Serum Male Adult Reference Range

3.5 - 7.2 mg/dL or 0.21 - 0.43 mmol/L.

Plasma or Serum Female Adult Reference Range

2.6 - 6.0 mg/dL or 0.16 - 0.36 mmol/L.

Plasma or Serum Child Reference Range

2.0 - 5.5 mg/dL or 0.12 - 0.33 mmol/L.

Urine, 24 h Adult Reference Range

250 - 750 mg/d or 1.5 - 4.4 mmol/d.

Hyperuricemia

Overproduction of uric acid in 25-30% of patients; exacerbated by purine-rich diet, drugs and alcohol.

Plasma Uric Acid Concentration

> 6.0 mg/dL.

Tophi

Occurs in severe cases; uric acid crystal deposits form in tissue, causing deformities.

Gout Symptoms

Pain and inflammation of joints; monosodium urate crystals will be deposited between joints causing it to become red, hot, tender and swollen.

Causes of Hyperuricemia

High uric acid in the blood; gout; chemotherapeutic treatment; increased catabolism of nucleic acids; renal disease.

Chemotherapeutic Treatment

Administered to patients undergoing chemotherapy for proliferative diseases such as leukemia, lymphoma, multiple myeloma, and polycythemia.

Allopurinol

Drug that inhibits xanthine oxidase; used for treatment to reduce uric acid production.

Uric Acid Nephrolithiasis

Formation of renal calculi; acidic urine forms insoluble uric acid precipitates.

Treatment for Renal Stones

Stones may be dissolved by alkalinization of urine, increased fluid intake, or intake of xanthine oxidase inhibitors.

Purine Metabolism Disorder

Lesch-Nyhan Syndrome; causes increased metabolism of cell nuclei resulting in overproduction of uric acid.

Increased Uric Acid Causes

Hemolytic or megaloblastic anemia; increased tissue catabolism due to starvation; decreased uric acid excretion.

Purine-Rich Diet

Includes liver, kidney, sweetbreads, and shellfish; patients with increased uric acid must avoid purine-rich food.

Tophi Lumps

Physical manifestation of deposits; bulging lumps will form at the joints.

Postmenopausal Women

Have increased urate concentration.

Age Group Diagnosis

Gout is usually diagnosed between 30-50 years old; found primarily in men.

Anchovies

A type of small fish often used in cooking.

Liver

An organ that processes nutrients and detoxifies substances.

Asparagus

A green vegetable known for its long, slender stalks.

Codfish

A popular fish used in various cuisines.

Sweetbreads

A culinary term for the thymus or pancreas of an animal.

Fava Beans

A type of broad bean used in cooking.

Haddock

A type of fish commonly used in fish and chips.

Brains

The organ responsible for processing information in the body.

Garbanzo Beans

Also known as chickpeas, used in various dishes.

Herring

A small, oily fish often eaten pickled.

Bacon

Cured meat from the belly of a pig.

Edamame (Soy)

Young soybeans often served as a snack or appetizer.

Mackerel

A type of fish known for its rich flavor.

Turkey

A large bird often eaten during Thanksgiving.

Mushroom

A type of fungus used in cooking.

Mussels

A type of shellfish commonly used in seafood dishes.

Veal

Meat from young cattle.

Peas

Small, round green seeds often used in cooking.

Sardines

Small, oily fish often canned.

Venison

Meat from deer.

Lentils

Small legumes often used in soups and salads.

Scallops

A type of shellfish known for its sweet flavor.

Beef

Meat from cattle.

Spinach

A leafy green vegetable rich in iron.

Trout

A type of freshwater fish.

Chicken

Meat from domesticated birds.

Cauliflower

A white vegetable in the cabbage family.

Crab

A type of shellfish with a hard shell.

Duck

A waterfowl often raised for its meat.

Lobster

A large marine crustacean known for its claws.

Ham

Cured meat from the hind leg of a pig.

Rare, X-linked genetic disorder

Affects males more and is associated with a complete deficiency of hypoxanthine guanine phosphoribosyltransferase (HGPRT).

HGPRT deficiency

When this enzyme is lacking, purine bases are not reutilized, causing high concentration of uric acid in blood and urine.

Compulsive self-mutilation

Behavior observed in patients with HGPRT deficiency, particularly affecting the head and hands.

Mental retardation

A condition often associated with HGPRT deficiency.

Involuntary muscular movements

A symptom presented in patients with HGPRT deficiency.

Gout

A form of arthritis characterized by severe pain and swelling, often due to high uric acid levels.

Glomerular Filtration Rate (GFR)

The rate at which ultrafiltrate is formed by the glomerulus, approximately 100-120 mL per minute.

Plasma creatinine concentration

A function of relative muscle mass, rate of creatine turnover, and renal function.

Creatinine

A waste product in the blood that is excreted in urine, used to assess kidney function.

Creatine

A compound that is 5% in plasma concentration and is involved in energy production.

Creatine

Synthesized primarily in the liver from arginine, glycine, and methionine, then transported to other tissues, such as muscle, where it is converted to creatine phosphate.

Creatine Phosphate

Loses phosphoric acid and creatine loses water to form the cyclic compound, creatinine.

GFR

Glomerular filtration rate, calculated using the formula GFR = V/t.

Volume of Filtered Plasma (V)

The volume of plasma that is filtered through the glomeruli per unit time.

Time (t)

The duration over which the volume of filtered plasma is measured.

Creatinine

Formed from creatine and creatine phosphate in muscle when creatine loses water and creatine phosphate loses phosphoric acid; excreted into plasma at a constant rate related to muscle mass.

Creatinine Clearance (CrCl)

A measure of the amount of creatinine eliminated from the blood by the kidneys, providing a reasonable approximation of the GFR, involving a 24-hour urine collection to measure creatinine excretion.

Creatinine Clearance Formula

CrCl = (UCr VU) / (PCr t), where UCr = urine creatinine concentration (mg/dL), PCr = plasma creatinine concentration (mg/dL), VU = urine volume (mL), and t = time (minutes).

Enzymatic Method

A method involving coupled enzymatic reactions to measure the concentration of substances.

Jaffe Reaction

A chemical method for measuring creatinine that lacks sensitivity.

Creatininase Method

A series of reactions catalyzed by enzymes including creatininase, creatine kinase, pyruvate kinase, and lactate dehydrogenase, producing NAD+ measured as a decrease in absorbance.

Creatininase: Hydrogen Peroxidase Method

Uses enzymes like creatininase, creatinase, sarcosine oxidase, and peroxidase, adapted for dry slide analyzers, and is more specific than the Jaffe method.

Isotope Dilution Mass Spectrometry (IDMS)

A highly specific detection method using isotopically labeled compounds and accepted as a reference method.

Specimen Requirements

Includes serum, plasma, or urine; fasting not required, and specific sample handling instructions for different methods.

Urine Volume (VU)

The total volume of urine collected for measurement.

Urine Concentration of Substance (US)

The concentration of a specific substance in the urine.