Introduction to laboratory profiling and laboratory profiling of the urinary system: A case oriented approach (Proceedings)
Biochemical profiling may be defined as the use of multiple blood chemistry determinations to assess the health status of various organ systems simultaneously. Biochemical profiling rapidly has become a major diagnostic aid for the practicing veterinarian for several reasons. First, a more educated clientele has come to expect increased diagnostic sophistication. Secondly, the advent of high-volume clinical pathology laboratories has resulted in low prices that make profiling in veterinary practice feasible and convenient. In addition, improved technology has resulted in the development of procedures that can be used to obtain accurate analyses on microsamples of serum. Such procedures offer obvious advantages to veterinarians, who in the past were hindered by requirements for large sample size.
Although biochemical profiling offers exciting potential, it is not a panacea. Since standard chemical screens provide 12 to 30 test results, interpretation of data may be extremely complex. Interpretation is often clouded by the fact that perfectly normal animals may have, indeed, are expected to have, an occasional abnormal test result. It is estimated that in a panel of 12 chemistry tests, approximately 46% of all normal subjects will have at least one abnormal test result. Such abnormalities do not reflect inaccuracies in laboratory test procedures but rather the way in which reference (or normal) values are determined. In order to establish the "normal range" for a given test, the procedure is performed on samples from a large population of clinically normal individuals. A mean and a standard deviation are determined. The reference values are then defined as those values falling within two standard deviations above and below the mean. Since two standard deviations above and below the mean only include 95% of all determined values, 5% of the values obtained from a normal population are by this definition abnormal.
It is important to realize that determination of reference values in the manner described above assumes a Gaussian or bell-shaped distribution for measured values. Additional problems with the establishment of reference values can be expected if Gaussian distributions are not present. In these instances, population distributions must be normalized to a bell-shaped distribution before reference values are established.Just as healthy individuals may have occasional abnormal test results, so can individuals with severe organ disease have test results that are within the reference intervals. For example, elevated serum alanine aminotransferase (ALT) levels long have been considered important indicators of liver disease in dogs. However, ALT levels will only be elevated under specific circumstances. ALT is an enzyme normally found in the cytosol of hepatocytes. Consequently, serum levels will only be elevated in conditions where there is increased permeability of plasma membranes. In more chronic liver disease, plasma membrane permeability is often normal. Additionally, ALT levels reflect the number of hepatocytes with leaky membranes; therefore, marked elevations are more commonly seen in diffuse than in localized liver disease.
ALT levels also will vary with the stage of the disease when the sample is collected. ALT has a circulatory half-life of two to four days; therefore, a two-fold elevation in ALT due to acute liver necrosis may be expected to have returned to the normal range within two days.
The clinician must also be aware that abnormalities in one organ system may cause abnormalities in chemistry test results that are used primarily to indicate disease in a different organ system. For example, elevated serum amylase levels are used primarily as indicators of pancreatic disease. However, amylase normally is excreted by the kidney as a part of glomerular filtrate. Consequently, anything that reduces glomerular filtration may result in elevated serum amylase levels.
Hopefully, the preceding paragraphs have succeeded in illustrating some of the more important difficulties encountered in the interpretation of clinical chemistry data. It is apparent that a single test should never be used to assess the total health status of an organ. It is equally apparent that one must understand the factors affecting a given test result, such as the causes of elevations, circulating half-lives, and routes of excretion. Then too, interactions between different organ systems and their effects upon test results must be considered. In the final analysis, it is apparent that only through systematic assessment of chemistry data can misinterpretation and confusion be avoided. A final point to consider is that chemistry profiling should not be undertaken without simultaneous evaluation of a complete blood count (CBC) and urinalysis.
Urinary system introduction
The kidney, like the liver, performs a variety of functions of major importance to the maintenance of normal homeostasis. It is involved in the excretion of wastes and the regulation of acid-base balance, electrolyte balance, and state of hydration.
The performance of these functions depends upon both normal glomerular filtration and normal renal tubular integrity. The primary renal panel assesses both. It is important to note that urinalysis, although not a part of our large chemistry profile, is an essential part of the primary renal panel. The secondary renal panel is primarily designed to evaluate changes that may occur secondary to renal disease.