AUBF: Physical Examination

• The physical examination of urine includes the determinationof the urine color, clarity, and specific gravity.
• Observation of these characteristics provides preliminary information concerning disorders such as glomerular bleeding, liver disease, inborn errors of metabolism, and urinary tract infection.
• Measurement of specific gravity aids in the evaluation of renal tubular function.
• The yellow color of urine is caused by the presence of a pigment, which Thudichum named Urochrome in 1864.
• Urochrome is a product of endogenous metabolism, and under normal conditions the body produces it at a constant rate.
• A dilute urine will be pale yellow and a concentrated specimen will be dark yellow.
• Two additional pigments, uroerythrin and urobilin, are also present in the urine in much smaller quantities, and contribute little to the color of normal, fresh urine.
• The presence of uroerythrin, a pink pigment, is most evident in specimens that have been refrigerated, resulting in the precipitation of amorphous urates.
• Urobilin, an oxidation product of the normal urinary constituent urobilinogen, imparts an orange-brown color to urine that is not fresh.
• Dark yellow or amber urine can be caused by the presence of the abnormal pigment bilirubin.
• If bilirubin is present, it will be detected during the chemical examination; however, its presence is suspected if yellow foam appears when the specimen is shaken.
• Normal urine produces only a small amount of rapidly disappearing foam when shaken, and a large amount of white foam indicates an increased concentration of protein.
• A urine specimen that contains bilirubin may also contain hepatitis virus.
• Photo-oxidation of bilirubin imparts a yellow-green color to the urine caused by the presence of biliverdin.
• Also frequently encountered in the urinalysis laboratory is the yellow-orange specimen caused by the administration of phenazopyridine to people who have urinary tract infections.
• Specimens containing phenazopyridine produce a yellow foam when shaken, which could be mistaken for bilirubin.
• One of the most common causes of abnormal urine color is the presence of blood.
• Red blood cells (RBCs) remaining in an acidic urine for several hours produce a brown urine due to the oxidation of hemoglobin to methemoglobin.
• A fresh brown urine containing blood may also indicate glomerular bleeding resulting from the conversion of hemoglobin to methemoglobin.
• Hemoglobin and myoglobin, produce a red urine and result in a positive chemical test result for blood.
• When RBCs are present, the urine is red and cloudy; however, if hemoglobin or myoglobin is present, the specimen is red and clear.
• Distinguishing between hemoglobinuria and myoglobinuria may be possibly by examining the patient’s plasma.
• Hemoglobinuria resulting from the in vivo breakdown of RBCs is accompanied by red plasma. Breakdown of skeletal muscle produces myoglobin.
• Fresh urine containing myoglobin frequently exhibits a more reddish-brown color than does urine containing hemoglobin.
• Urine specimens containing porphyrins also may appear red, resulting from the oxidation of porphobilinogen to porphyrins. They are often referred to as having the color of port wine.
• Many medications, including rifampin, phenolphthalein, phenindione, and phenothiazines, produce red urine.
• Urine specimens that turn brown or black on standing may contain melanin or homogentisic acid.
• Melanin is an oxidation product of the colorless pigment, melanogen, produced in excess when a malignant melanoma is present.
• Homogentisic acid, a metabolite of phenylalanine, imparts a black color to alkaline urine from persons with the inborn-error of metabolism, called alkaptonuria.
• Pathogenic causes of blue/green urine are limited to bacterial infections, including urinary tract infection by Pseudomonas species.
• The medications methocarbamol, methylene blue, and amitriptyline may cause blue urine.
• Clarity is a general term that refers to the transparency or turbidity of a urine specimen. In routine urinalysis, clarity is determined in the same manner that ancient physicians used: by visually examining the mixed specimen while holding it in front of a light source.
• Common terminology used to report clarity includes clear, hazy, cloudy, turbid, and milky.
• Precipitation of amorphous phosphates and carbonates may cause a white cloudiness.
• The presence of squamous epithelial cells and mucus, particularly in specimens from women, can result in a hazy but normal urine.
• Refrigerated specimens frequently develop a thick turbidity caused by the precipitation of amorphous phosphates, carbonates, and urates.
• Amorphous phosphates and carbonates produce a white precipitate in urine with an alkaline pH, whereas amorphous urates produce a precipitate in acidic urine that resembles pink brick dust due to the presence of uroerythrin.
• The most commonly encountered pathologic causes of turbidity in a fresh specimen are RBCs, WBCs, and bacteria caused by infection.
• The specific gravity of the plasma filtrate entering the glomerulus is 1.010.
• The term isosthenuric is used to describe urine with a specific gravity of 1.010. Specimens below 1.010 are hyposthenuric, and those above 1.010 are hypersthenuric.