Urinalysis: Specific Gravity Calculation

Urinalysis

• Specific gravity measures the ability of the kidneys to reabsorb essential chemicals and water.
• For every g/dL of CHON, deduct 0.003 from the specific gravity reading.
• For every g/dL of glucose, deduct 0.004 from the specific gravity reading.
• For every 3°C drop in calibration temperature, deduct 0.001 from the specific gravity reading.
• For every 3°C above the calibration temperature, add 0.001 to the specific gravity reading.
• Calibration temperature is usually 20°C.

Clinical Microscopy

• Refractometry measures specific gravity by measuring the refractive index.
• Refractive index is a comparison of the velocity of sound waves in air and in the urine sample.
• Electric current is applied to the coil, causing a sound wave to pass through the urine sample.
• The microprocessor measures the change in sound wave frequency and converts it to specific gravity.

Blood Reagent Strip

• Pseudoperoxidase activity of hemoglobin is the principle of the blood reagent strip.
• Reagents used are peroxide, tetramethylbenzidine, and buffer.
• Reaction: H2O2 + chromogen –Hgb Peroxidase→ oxidized chromogen (yellow to green to blue color*) + H2O.
• Sensitivity: Multistix (5-20 RBCs/mL, 0.015-0.062 mg/dL hgb), Chemstrip (5 RBCs/mL, hgb corresponding to 10 RBCs/mL).
• Correlation with other tests: protein, microscopic examination.

Bilirubin

• Formation of bilirubin: RES (RBC → hgb → heme → protoporphyrin → biliverdin → B1) → circulation (B1 coupled with albumin) → liver (B1 conjugated with glucoronic acid to form B2) → intestines (B2 converted to urobilinogen by intestinal bacteria).
• Clinical significance of bilirubinuria: early indicator of liver disease, conjugated bilirubin appears in urine when there is bile duct obstruction or liver damage.
• Used in determining the cause of jaundice.

Ascorbic Acid

• Urine tests for ascorbic acid have been used as an indication of adequate ascorbic acid therapy.
• Large quantities of ascorbic acid may occasionally be found in the urine of individuals taking therapeutic doses of vitamin C or other preparations containing ascorbic acid.
• Reagent: methylene green, which is reduced to its colorless form with ascorbic acid.
• Neutral red provides a background color, and the overall color changes from blue to purple at levels of 150 mg/dL.

Microscopic Examination of Urine

• Used to detect and identify insoluble materials in urine.
• Must include quantitation of elements present.
• Some elements are considered normal unless present in increased amounts.
• Slide examined first under LPO, setting changed to HPO for identification.
• LPO scanning of cover-slip perimeter is recommended.
• For detection of casts, use reduced light when examining unstained sediment by bright-field microscope.

Reporting the Microscopic Examination

• May vary among laboratories but must be consistent within a particular laboratory system.
• Routine reporting includes:
  • Casts: per 10 LPFs.
  • RBCs and WBCs: per 10 HPFs.
  • Epithelial cells, crystals, and other elements: semi-quantitative terms (rare, few, moderate, many).

Oculars/Eyepiece Lens

• Responsible for further magnification and resolution (ability to visualize fine details).
• Coarse- and fine-adjustment knobs.
• Iris diaphragm: controls the amount of light that will pass through the specimen.
• Rheostat: controls the intensity of light.
• Image in the microscope is upside down and reversed.

Microscopy

• Kohler illumination is a technique to optimize light quality and sharpness by aligning and adjusting each component of the optical system.
• Adjustments done to provide optimal viewing of illuminated objects.
• Steps:
  • Place a slide on the stage and focus using LPO with the condensers raised.
  • ...