Urine Analysis: Physical and Chemical Characteristics of Normal Urine

Urine analysis infers valuable information in a variety of ailments. Physical characteristics of urine have been used as diagnostic and prognostic tool from the time immemorial by the health physicians. We know that the major functions of kidneys are:

1. Removal of water not needed by the body fluids, the amount depending on the balance between glomerular filtrate and he degree of tubular reabsorption;
2. The excretion of certain substances normally present in the plasma when their concentration rises above a certain level;
3. The selective reabsorption of substances such as glucose which are of value to the body;
4. The excretion of useless substances; and
5. Regulation of acid base balance.

Disordered renal function may lead to a change in the volume of the urine excreted per day along with remarkable changes in its physical and chemical properties and microscopic contents. Urine analysis is the very first investigation of diagnostic importance not only in renal disorders but also in other diseases like diabetes, liver disease, jaundice etc. In diagnostic pathology the extent of abnormalities could only be understood in comparison with the reference values obtained from similar investigations in normal individuals. Hence, it is important to have an understanding of normal parameters of physical and chemical characteristics of urine.

Characteristics of normal urine:

1. Quantity: The quantity averages 1500 to 2000 ml in an adult man daily. It may vary with the amount of fluid taken. In fact it is linked with the protein metabolism; higher is the protein intake higher will be the urinary output since the urea produced from the protein needs to be flushed out from the body. Higher is the urea production in the body, the higher is the volume of urine to excrete it.
2. Color: The color should be clear pale amber without any deposits. However, a light flocculent cloud of mucus may sometimes be seen floating in the normal urine.
3. Specific gravity: It varies from 1.010 to 1.025. Specific gravity is determined with urinometer.
4. Odor: The odor is aromatic.
5. Reaction: The reaction of normal urine is slightly acidic with an average pH of 6.0.

Composition of normal urine: Urine is mainly composed of water, urea and sodium chloride. I an adult taking about 100 g protein in 24 hours, the composition of urine is likely to be as follows:

1. Water: Near about 96%
2. Solids: About 4% (urea 2% and other metabolic products 2%. Other metabolic products include: uric acid, creatinine, electrolytes or salts such as sodium chloride, potassium chloride and bicarbonate).

• Urea is one of the end products of protein metabolism. It is prepared from the deaminated amino-acid in the liver and reach the kidneys through blood circulation (The normal blood urea level is 20-40 mg/dl). About 30 gram urea is excreted by the kidneys daily.
• Uric Acid: The normal level of uric acid in blood is 2 to 6 mg/dl and about 1.5 to 2 gram is excreted daily in urine.
• Creatinine: Creatinine is the metabolic waste of creatin in muscle. Purine bodies, oxalates, phosphates, sulphates and urates are the other metabolic products.
• Electrolytes or salts such as sodium chloride and potassium chloride are also excreted in the urine to maintain the normal level in blood. These are the salts which are the part of our daily diet and are always taken in excess and need to be excreted to maintain normal physiological balance.