Urinalysis or urine routine examination is a very simple test and easily accessible everywhere. It can give clue to a variety of underlying kidney diseases. Urinalysis consists of gross inspection, urine dipstick examination and urine microscopy.
How to obtain a urine specimen for urinalysis?
A clean container should be used for collection of urine to avoid abnormal results. The patient should be asked to clean the external genitalia prior to collecting the sample. A mid stream sample should be collected. If the sample is to be collected from a patient who is cathetrized, it should be collected from the catheter and not the urine bag. Urine examination should be carried out at room temperature within 2 hours of the sample collection otherwise it should be stored at 2 celsius to 8 celsius.
What does gross appearance of your urine indicate?
Urine becomes turbid when it contains alot of cells, proteins or bacteria.
Light colour indicates dilute urine while it is dark when concentrated as in patients with dehydration. Red or brown color urine can be caused by hematuria ( blood in urine), hemoglobinuria or myoglobinuria. Hemoglobinuria occurs in patients with hemolysis i.e. breakdown of red blood cells. Wheresas myoglobinuria occurs in patients with muscle breakdown as in crush injuries or certain diseases of tbe muscles as well as in snake bite. Patients who are taking certain drugs can also have reddish urine. The best example is patients taking rifampicin as part of anti-tuberculous therapy. Other drugs like phenytoin ( an antiepileptic drug) and ingestion of beet (beeturia) and senna can also cause reddish urine. Certain rare conditions like patients with porphyrea can notice brownish discoloration of their urine after exposure to sunlight for sometime.
This is a simple and rapid semiquantitative test. Dipstick test can be positive for leukocyte esterase. This is indicative of infection. Microscopy and cultures become mandatory to look for pus cells and bacteria in the sample. In certain certain conditions pus cells can be positive with negative culture. This is called as sterile pyuria. Renal tuberculosis, interstitial nephritis and nephrolithiasis or renal stones can all present with sterile pyuria.
Dipstick may show positive proteins. Dipsticks are specifically sensitive for albumin. Albumin when excreted in small quantity can not be detected by dipstick. Albumin excretion of 30 to 300 is an important clue to the diagnosis of diabetic kidney disease. Positive proteins on urinalysis may be suggestive of glomerular diseases especially nephrotic syndrome. Quantification of 24 urinary proteins is essential in patients suspected of having glomerular diseases. In certain diseases like myeloma where immunoglobulins are excreted in urine, urinalysis might be of little use as these proteins are not usually detected by dipstick methods.
Normal urinary PH ranges from 4.5 to 8 depending on systemic acid-base balance. PH monitoring is especially useful in patients with systemic acidosis. The normal body response is to excrete acid via kidneys in patients with systemic acidosis. In renal tubular acidosis, the kidneys are not able to excrete excess acid and fail to acidify urinary PH.
The concentration of urine by different osmotic particles directly affects its specific gravity. Simply specific gravity is the weight of urinary sample compared to an equal volume of distilled water. In patients with states of dehydration, its specific gravity increases. In diabetes insipidus, large volume of diluted urine is excreted because of the loss of concentrating ability of the kidneys. The specific gravity in diabetes insipidus thus is very low. A vague estimation of urinary osmolality can be done by multiplying the last two digits after the decimals with a factor of 30.
Positive glucose on urinary dipstick indicates spillage of glucose into urine. In normal persons the kidneys are able to reabsorb all filtered glucose back into the systemic circulation. If plasma glucose exceeds 180 mg/dl or 10 mmol/l, glucose can be detected in the urinary sample. In pregnancy this threshold may be lowered and patients may have glucosuria (glucose in urine) without raised plasma glucose. Other conditions in which plasma glucose is normal but the dipstick is positive for glucose include familial glucosuria, fanconi syndrome (defect in tubular reabsorption) and drugs like SGLT 2 inhibitors (dapagliflozin, sotagliflozin). Last but not the least, the most important cause of glucosuria is diabetes mellitus. Keeping in view the above conditions in mind and the normal physiological states where one can have positive dipstick for urinary glucose, diabetes mellitus should never be diagnosed based on positive dipstick for glucose alone.
Urine microscopy findings:
The next important part of urinalysis is microscopy. Microscopy can identify bacteria and cellular elements.
Red blood cells on microscopy:
Of the cellular elements, red blood cells in urine also called as hematuria can be frightening when it is visible to the naked eye. This is called as gross hematuria. Most of the causes of hematuria are benign. Blood in urine can come from the kidneys or anywhere in the urinary tract. Hematuria can be found normal in female patients who are menstruating so a repeat sample after some days is recommended. Persistent hematuria should always be investigated.
The first step in the workup of hematuria is to find out whether the blood is coming from the kidneys or elsewhere. Blood coming from the kidneys or glomerular hematuria may be accompanied by proteinuria and the shape of the red blood cells may be dysmorphic. Red blood cell casts are formed of proteins and red cells. Red cell casts are specifically found in glomerular diseases. Young patients may develop hematuria after a recent respiratory tract infection. This is called as Ig A nephropathy. In contrast to young patients, old patients with gross hematuria should be investigated for serious underlying diseases like bladder growth. Patients with bleeding disorders can also present with hematuria.
Pus in urine is a sign of infection especially when neutrophils are dominant. Other cells like eosinophils may be present in patients with interstitial nephritis which is usually secondary to ingestion of drugs like NSAIDs.
Red blood cell casts can be seen in glomerulonephritis. White blood cell casts suggests infection of the kidneys (pyelonephritis). Granular casts represent degenerated cellular casts and can be found in acute kidney injury (granular casts are characteristic of acute tubular necrosis). Hyaline casts are transparent and represent volume depletion or diuretic therapy. Waxy casts are very non specific. Whereas broad casts are usually found in advanced chronic kidney disease.
Crystaluria or crystals in urine can be a clue to the underlying disease. Magnesium phosphate crystals are found in infections. Whereas large number of uric acid crystals with acute kidney injury is consistent with tumor lysis syndrome.