Analysis of Urine

Analysis of Urine

In the normal adult, about 1200 ml of blood passes through the kidney each minute, exposing the plasma to the semi-permeable membrane of each functioning glomerulus. The ultra-filtrate that collects in Bowman’s capsule contains all of the substances of the plasma capable of passing through the membrane. Modification of this filtrate to produce excreted urine occurs in the tubules and collecting duct of the nephron. Threshold substances such as glucose and amino acids are reabsorbed. Waste materials such as creatinine, urea, uric acid, phosphates, and other materials are left in the filtrate to be excreted. The three principle factors affecting the composition of the excreted urine are nutritional status, the state of metabolic processes, and the ability of the kidney to selectively process the material presented to it.

Two of the most common assessments of kidney function are serum measurements of urea nitrogen (BUN) and creatinine.  Both of these substances are excreted entirely by the kidneys, so correlate well to kidney function.  However, it is important to consider that there are additional conditions that may also affect serum concentrations of these substances so, as always, results should be interpreted with consideration of the entire clinical picture.

Because urinalysis can screen for a variety of conditions and is a relatively easy specimen to obtain, it is a frequently performed assessment. Examination of both solute and urine sediment is done in the standard urinalysis. Among the most important conditions readily detected by chemical testing of urine solute are glycosuria, ketonuria, proteinuria, and the presence of the pigments bilirubin, urobilinogen, and hemoglobin. Proteinuria is probably the most common indication of renal disease. Microscopic examination of the urine sediment provides important information concerning the kidneys and urinary tract not readily obtainable in any other way. Urinary sediment can include red blood cells, white blood cells, epithelial cells, fat of biological origin, casts, bacteria, yeast, fungi, parasites, spermatozoa, crystals, and amorphous material. Some of these substances have pathological significance and some are not as clinically important.

In addition to the standard urinalysis, many other compounds can be measured in urine. For example, there is increased excretion of the organic acid formiminoglutamate (FIGLU) when folic acid is deficient. Similarly, xanthurenic acid increases when there is a lack of B6, which is necessary for its metabolism. There are many additional organic acids that can provide information with regard to nutritional deficiencies, dysbiosis, and inadequate detoxification. All of these metabolites are measured in urine. An overview of these organic acids is presented in this unit.  We will discuss many of these compounds in more detail as we progress through the course.

To understand how these organic acids can be helpful in identifying need for nutrient support, the diagram below may be helpful.  Note that the conversion of B to C is inhibited due to lack of a coenzyme. Therefore, if we know the coenzyme necessary for the reaction to occur, we can assume that providing that nutrient will enable the pathway to proceed in normal fashion.  This functional approach is often more sensitive than other assessments because it can identify those who may have increased needs due to lifestyle or genetic factors.