The parameters to be examined on physical examination of urine are listed below.
- Specific Gravity
Volume of only the 24-hr specimen of urine needs to be measured and reported. The average 24-hr urinary output in adults is 600-2000 ml. The volume varies according to fluid intake, diet, and climate. Abnormalities of urinary volume are as follows:
- Polyuria means urinary volume > 2000 ml/24 hours. This is seen in diabetes mellitus (osmotic diuresis), diabetes insipidus (failure of secretion of antidiuretic hormone), chronic renal failure (loss of concentrating ability of kidneys) or diuretic therapy.
- Oliguria means urinary volume < 400 ml/24 hours. Causes include febrile states, acute glomerulonephritis (decreased glomerular filtration), congestive cardiac failure or dehydration (decreased renal blood flow).
- Anuria means urinary output < 100 ml/24 hours or complete cessation of urine output. It occurs in acute tubular necrosis (e.g. in shock, hemolytic transfusion reaction), acute glomerulonephritis, and complete urinary tract obstruction.
Normal urine color in a fresh state is pale yellow or amber and is due to the presence of various pigments collectively called urochrome. Depending on the state of hydration urine may normally be colorless (over hydration) or dark yellow (dehydration). Some of the abnormal colors with associated conditions are listed in Table 819.1.
|Colorless||Dilute urine (diabetes mellitus, diabetes insipidus, overhydration)|
|Red||Hematuria, Hemoglobinuria, Porphyria, Myoglobinuria|
|Dark brown or black||Alkaptonuria, Melanoma|
|Yellow-green or green||Biliverdin|
|Deep yellow with yellow foam||Bilirubin|
|Orange or orange-brown||Urobilinogen/Porphobilinogen|
|Red or orange fluorescence with UV light||Porphyria|
Normal, freshly voided urine is clear in appearance. Causes of cloudy or turbid urine are listed in Table 819.2. Foamy urine occurs in the presence of excess proteins or bilirubin.
|1. Amorphous phosphates||White and cloudy on standing in alkaline urine||Disappear on addition of a drop of dilute acetic acid|
|2. Amorphous urates||Pink and cloudy in acid urine||Dissolve on warming|
|3. Pus cells||Varying grades of turbidity||Microscopy|
|4. Bacteria||Uniformly cloudy; do not settle at the bottom following centrifugation||Microscopy, Nitrite test|
Freshly voided urine has a typical aromatic odor due to volatile organic acids. After standing, urine develops ammoniacal odor (formation of ammonia occurs when urea is decomposed by bacteria). Some abnormal odors with associated conditions are:
- Fruity: Ketoacidosis, starvation
- Mousy or musty: Phenylketonuria
- Fishy: Urinary tract infection with Proteus, tyrosinaemia.
- Ammoniacal: Urinary tract infection with Escherichia coli, old standing urine.
- Foul: Urinary tract infection
- Sulfurous: Cystinuria.
SPECIFIC GRAVITY (SG)
This is also called as relative mass density. It depends on amount of solutes in solution. It is basically a comparison of density of urine against the density of distilled water at a particular temperature. Specific gravity of distilled water is 1.000. Normal SG of urine is 1.003 to 1.030 and depends on the state of hydration. SG of normal urine is mainly related to urea and sodium. SG increases as solute concentration increases and decreases when temperature rises (since volume expands with rise in temperature).
SG of urine is a measure of concentrating ability of kidneys and is determined to get information about this tubular function. SG, however, is affected by proteinuria and glycosuria.
Causes of increase in SG of urine are diabetes mellitus (glycosuria), nephrotic syndrome (proteinuria), fever, and dehydration.
Causes of decrease in SG of urine are diabetes insipidus (SG consistently between 1.002-1.003), chronic renal failure (low and fixed SG at 1.010 due to loss of concentrating ability of tubules) and compulsive water drinking.
Methods for measuring SG are urinometer method, refractometer method, and reagent strip method.
1. Urinometer method:
This method is based on the principle of buoyancy (i.e. the ability of a fluid to exert an upward thrust on a body placed in it). Urinometer (a hydrometer) is placed in a container filled with urine (Figure 819.1A). When solute concentration is high, upthrust of solution increases and urinometer is pushed up (high SG). If solute concentration is low, urinometer sinks further into the urine (low SG).
Accuracy of a urinometer needs to be checked with distilled water. In distilled water, urinometer should show SG of 1.000 at the temperature of calibration. If not, then the difference needs to be adjusted in test readings taken subsequently.
The method is as follows:
- Fill a measuring cylinder with 50 ml of urine.
- Lower urinometer gently into the urine and let it float freely.
- Let urinometer settle; it should not touch the sides or bottom of the cylinder.
- Take the reading of SG on the scale (lowest point of meniscus) at the surface of the urine.
- Take out the urinometer and immediately note the temperature of urine with a thermometer.
Correction for temperature: Density of urine increases at low temperature and decreases at higher temperature. This causes false reading of SG. Therefore, SG is corrected for difference between urine temperature and calibration temperature. Check the temperature of calibration of the urinometer To get the corrected SG, add 0.001 to the reading for every 3°C that the urine temperature is above the temperature of calibration. Similarly subtract 0.001 from the reading for every 3°C below the calibration temperature.
Correction for dilution: If quantity of urine is not sufficient for measurement of SG, urine can be appropriately diluted and the last two figures of SG are multiplied by the dilution factor.
Correction for abnormal solute concentration: High SG in the presence of glycosuria or proteinuria will not reflect true kidney function (concentrating ability). Therefore it is necessary to nullify the effect of glucose or proteins. For this, 0.003 is subtracted from temperature-corrected SG for each 1 gm of protein/dl urine and 0.004 for every 1 gm of glucose/dl urine.
2. Refractometer method:
SG can be precisely determined by a refractometer, which measures the refractive index of the total soluble solids. Higher the concentration of total dissolved solids, higher the refractive index. Extent of refraction of a beam of light passed through urine is a measure of solute concentration, and thus of SG. The method is simple and requires only 1-2 drops of urine. Result is read from a scale or from digital display.
3. Reagent strip method:
Reagent strip (Figure 819.1B) measures the concentration of ions in urine, which correlates with SG. Depending on the ionic strength of urine, a polyelectrolyte will ionize in proportion. This causes a change in color of pH indicator (bromothymol blue). Also read: URINE STRIP TEST — UNDERSTANDING ITS LIMITATIONS.
REACTION AND pH
The pH is the scale for measuring acidity or alkalinity (acid if pH is < 7.0; alkaline if pH is > 7.0; neutral if pH is 7.0). On standing, urine becomes alkaline because of loss of carbon dioxide and production of ammonia from urea. Therefore, for correct estimation of pH, fresh urine should be examined.
There are various methods for determination of reaction of urine: litmus paper, pH indicator paper, pH meter, and reagent strip tests.
- Litmus paper test: A small strip of litmus paper is dipped in urine and any color change is noted. If blue litmus paper turns red, it indicates acid urine. If red paper turns blue, it indicates alkaline urine (Figure 819.2A).
- pH indicator paper: Reagent area (which is impregnated with bromothymol blue and methyl red) of indicator paper strip is dipped in urine sample and the color change is compared with the color guide provided. Approximate pH is obtained.
- pH meter: An electrode of pH meter is dipped in urine sample and pH is read off directly from the digital display. It is used if exact pH is required.
- Reagent strip test: The test area (Figure 819.2B) contains polyionic polymer bound to H+; on reaction with cations in urine, H+ is released causing change in color of the pH-sensitive dye. Also read: URINE STRIP TEST — UNDERSTANDING ITS LIMITATIONS.
Normal pH range is 4.6 to 8.0 (average 6.0 or slightly acidic). Urine pH depends on diet, acid base balance, water balance, and renal tubular function.
Acidic urine is found in ketosis (diabetes mellitus, starvation, fever), urinary tract infection by Escherichia coli, and high protein diet. Alkaline urine may result from urinary tract infection by bacteria that split urea to ammonia (Proteus or Pseudomonas), severe vomiting, vegetarian diet, old ammoniacal urine sample and chronic renal failure.
Determining pH of urine helps in identifying various crystals in urine. Altering pH of urine may be useful in treatment of renal calculi (i.e. some stones form only in acid urine e.g. uric acid calculi; in such cases urine is kept alkaline); urinary tract infection (urine should be kept acid); and treatment with certain drugs (e.g. streptomycin is effective in urinary tract infection if urine is kept alkaline). In unexplained metabolic acidosis, measurement of urine pH is helpful in diagnosing renal tubular acidosis; in renal tubular acidosis, urine pH is consistently alkaline despite metabolic acidosis.