3. Transparency.—Freshly passed normal urine is clear. Upon standing, a faint cloud of mucus, leukocytes, and epithelial cells settles to the bottom. Abnormal cloudiness is usually due to presence of phosphates, urates, pus, blood, or bacteria.
Amorphous phosphates are precipitated in neutral or alkaline urine. They form a white cloud and sediment which disappear upon addition of an acid.
Amorphous urates are precipitated only in acid urine. They form a white or pink cloud and sediment ("brick-dust deposit") which disappear upon heating.
Pus resembles amorphous phosphates to the naked eye. Its nature is easily recognized with the microscope, or by adding a strong solution of caustic soda to the sediment, which is thereby transformed into a gelatinous mass (Donné's test).
Blood gives a reddish or brown, smoky color, and may be recognized with the microscope or by tests for hemoglobin.
Bacteria, when present in great numbers, give a uniform cloud which cannot be removed by ordinary filtration. They are detected with the microscope.
The cloudiness of decomposing urine is due mainly to precipitation of phosphates and multiplication of bacteria.
4. Reaction.—Normally, the mixed twenty-four-hour urine is slightly acid in reaction, the acidity being due to acid salts, not to free acids. Individual samples may be slightly alkaline, especially after a full meal. The reaction is determined by means of litmus paper.
Acidity is increased after administration of certain drugs, and whenever the urine is concentrated from any cause, as in fevers. A very acid urine may cause frequent micturition because of its irritation. This is often an important factor in the troublesome enuresis of children.
The urine always becomes alkaline upon long standing, owing to decomposition of urea with formation of ammonia. If markedly alkaline when voided, it usually indicates such "ammoniacal decomposition" in the bladder, which is the rule in chronic cystitis, especially that due to paralysis or obstruction. Alkalinity due to ammonia (volatile alkalinity) can be distinguished by the fact that litmus paper turned blue by the urine again becomes red upon gentle heating. Fixed alkalinity is due to alkaline salts, and is often observed during frequent vomiting, after the crisis of pneumonia, in various forms of anemia, after full meals, and after administration of certain drugs, especially salts of vegetable acids.