Chemical Examination of Urine

pH

  • Normal pH range: 4.5-8.0

    • First morning specimen is slightly acidic: 5.0-6.0

  • A pH > 8.5 indicates an old specimen

  • The pH reaction is a double-indicator system

    • Methyl red

    • Bromthymol blue

  • False-negatives can be caused from run-over from protein pad

Protein

  • Protein is most indicative of renal disease

  • Albumin is most sensitive protein when measuring

  • Tetrabromphenol blue is used in test strip

    • Detects color change at pH of 3

  • False-positives can be caused by highly alkaline urine

  • False-negatives can be caused by high salt concentrations

  • Proteinuria - abnormal amounts of protein in the urine (30mg/dL, 300mg/24h)

    • Bence Jones protein is protein found urine associated with multiple myeloma

      • Precipitates when heated to 60 C

    • Glomerular proteinuria is abnormal substances deposited on the glomerular membrane that cause damage

    • Microalbuminuria is albumin found in the urine associated with diabetic nephropathy and cardiovascular disease

      • Used to monitor patients with diabetes mellitus

    • Orthostatic proteinuria is present in samples collected when the donor is in a vertical position

    • Tubular proteinuria occurs when the renal tubules can not properly reabsorb filtered proteins

Glucose

  • Glucose is the most frequent chemical analysis performed on urine and is the major screening test for diabetes mellitus, renal tubular damage, and head injury

  • The chemical reaction for glucose testing is a double sequential enzyme system

    • Glucose oxidase

    • Peroxidase

  • False-positives can be caused by bleach

  • False-negatives can be caused by ascorbic acid, high SG, and old specimens

Ketones

  • Ketones are present in urine when body stores of fat must be metabolized to supply energy

  • The primary reagent in the ketone strip reaction is sodium nitroprusside

    • Measure primarily acetoacetic acid

  • False-positives can be caused by Levodopa (medication for movement disorders) and highly pigmented urines

  • False-negatives can be caused be improperly preserved specimens

Blood

  • Hematuria - intact RBCs found in urine; cloudy red urine

    • Can be caused by renal damage and anticoagulants

  • Hemoglobinuria - product of RBC destruction; clear red urine

    • Can be caused by lysis of red blood in dilute, alkaline urine

  • Hemosiderin - yellow brown granules in sediment

  • Myoglobinuria - heme-containing protein in muscle tissue present in urine, clear red/brown urine

    • Caused by rhabdomyolysis (muscle destruction)

  • The principle of the blood reagent strip test is pseudoperoxidase activity of hemoglobin

    • Tetramethylbenzidine reagent

  • False-positives can be caused by bleach and menstrual contamination

  • False-negatives can be caused by ascorbic acid, ketones, and high SG

Bilirubin

  • Bilirubin is the degradation product of hemoglobin and is used as an early indicator of liver disease

  • The principle reaction in the reagent strip is a diazo reaction

  • False-positives can be caused by pigmented urine and medications

  • False-negatives can be caused by old specimens, ascorbic acid, exposure to light, and nitrite

Urobilinogen

  • Urobilinogen is converted from bilirubin in the intestine and reabsorbed into the liver

    • Only trace amounts commonly found in urine

  • Increased levels could be indication of liver disorders, hepatitis, and cirrhosis

  • Hemolytic disorders will have:

    • Negative bilirubin

    • Positive urobilinogen

  • The reagent used in the reagent strip reaction for urobilinogen is Ehrlich’s reagent

  • False-positives can be caused by some medications

  • False-negatives can be caused by nitrite

Nitrite

  • Nitrite is used for rapid screening test for UTIs

  • The reaction that occurs during the reagent strip reaction for nitrite is Griess’s reaction

  • False-positives can be caused by old specimens and highly pigmented urines

  • False-negatives can be caused by ascorbic acid and bacteria

Leukocytes Esterase

  • LE testing detects the presence of esterase in the granulocytes and monocytes which indicated lysed leukocytes

  • LE testing is used to detect UTIs and inflammation

  • During the reagent strip reaction for LE testing, acid esterase forms aromatic compound on the pad which then reacts with diazonium salt to from a purple color change if positive

    • Longest reaction of all the tests (2 minutes)

  • False-positives can be caused by bleach and highly pigmented urine

  • False-negatives can be caused by high concentrations of glucose and protein, ascorbic acid, and high SG

Specific Gravity

  • The principle for the SG strip reagent test is a pK change of polyelectrolyte using bromthymol blue

  • False-positives can be caused by protein

  • False-negatives can be caused by alkaline urine (>6.5)