Biochemical Tests Specific To Diagnose Renal Diseases
Urinary Phase Contrast Microscopy
A General Overview
Normal urine does not show a positive result by the heat test or sulphosalicylic acid test, though minute quantities can be detected by chemical tests. Presence of demonstrable proteins is abnormal. Proteinuria may result from renal disorders, lesions in the urinary tract or extrarenal causes like cardiac failure and fever. Gross proteinuria associated with the presence of casts is invariably due to a renal disease. Presence of blood in urine gives rise to considerable amounts of protein in urine and this has to be excluded before attributing proteinuria to a primary renal lesion. The quantity of proteins can be estimated.
Gross proteinuria (above 3.5g in 24 hours or 2g/m2 body surface) is diagnostic of nephrotic syndrome. Serial quantitative estimation of urinary proteins helps in monitoring the progress of nephrotic syndrome.
The estimation of excretory products such as urea, creatinine, uric acid and electrolytes such as sodium, potassium, calcium and phosphate are useful parameters to assess the capacity of the kidney to eliminate them.
Presence of glucose in urine often suggests a diagnosis of diabetes mellitus. Rarely, glucose may appear in urine in the presence of normal blood sugar and this occurs in renal glycosiuria. The conventional Benedict’s test for reducing substances may be positive in lactosuria, fructosuria or following intake of drugs.
Microscopic examination of the urinary deposits gives valuable information. Normal urine when centrifuged and examined, shows a few erythrocytes, leukocytes and occasional hyaline casts. Presence of numerous erythrocytes in uncentrifuged urine suggest the presence of blood. This is seen in acute glomerulonephritis, infections, tumours, renal infarction, ulcerations of the urinary tract and bleeding diathesis. Presence of leukocytes suggest active inflammation. Pyuria is a marked feature of urinary tract infection. Presence of leukocytes in all high power fields in uncentrifuged urine is a very suggestive feature of urinary infection.
Casts And Pathogens In Urine
Casts are formed from Tamm Horsfall proteins secreted by the tubular cells and the presence of large number casts indicates renal parenchymal disease. Since the casts are likely to be packed and disrupted by high speed centrifugation, it is advisable to use slow speed centrifugation to prepare the urinary deposit. The casts may be cellular (both RBC casts and leukocyte casts) granular or hyaline. Cellular casts indicate active inflammation. Granular casts in large numbers suggest a chronic inflammatory process. Renal tubular epithelial cells may be seen in the casts in tubular disorders.
Gram-staining and acid-fast staining can be done with the urinary deposit to identify bacterial pathogens. Since saprophytes and contaminants overgrow in stored urine, these tests should be carried out in fresh urine, collected with care to avoid external contamination. If there is likely to be delay in transport to the laboratory, the urine should be preserved with boric acid (1.8%) which prevents overgrowth of the contaminant organisms (to give a final concentration of 1.8%).
A clean catch mid-stream specimen of urine for microbiological examination is generally satisfactory. Urinary catherisation should not be routinely performed for this purpose, for fear of introducing infection. In addition to culture, quantitative determination of the colony count helps in deciding whether the organisms exist as contaminants or as active invaders. Bacterial count below 104/ml is suggestive of contaminants whereas those above 105/ml is in favour of active infection. For doing colony counts and to facilitate culture, dipsticks coated with the culture media are available commercially. These can be directly inoculated by immersion into the urine and incubated straight away.
© 2014 Funom Theophilus Makama