What are biochemical tests?
Biochemical tests play an essential role in infectious disease diagnosis, screening, prognosis, and treatment. Screening may be advisable for at-risk groups and for checking disease prevalence in a given population.
A physician begins a diagnosis by examining a person’s symptoms. Samples of blood, urine, feces, and tissues may be collected. The samples are sent to various departments in a medical laboratory for examination. These lab departments include bacteriology (culturing), immunology, and pathology.
Biochemistry departments aid in identifying pathogenic species or in distinguishing organisms from other species. Biochemical tests detect distinctive differences in metabolism of a species. These metabolic differences result in the formation of acid, gas, or other chemical products that can be detected by color changes or other means.
Many tests are named according to the enzyme active in the test; the enzyme names end with the letters ase. An evaluation of test results, together with a person’s clinical history, can lead to a prognosis, or a prediction of the course or outcome of the disease. Biochemical tests can also be important during the treatment phase to monitor changes in body metabolism or function.
Bacteria can be divided into two physiological groups depending on whether they retain a Gram’s stain or not. These bacteria are either gram-positive or gram-negative. Bacteria can assume various shapes,such as spherical (cocci) or rodlike (bacilli). Biochemical tests can be classified into three categories according to the characteristics of the bacteria being tested: gram-negative bacilli, gram-positive cocci, and gram-negative cocci.
The Enterobacteriaceae is a large family of gram-negative rods that inhabits the intestinal tract. Most Enterobacteriaceae are harmless normal flora of the intestines, but some can become pathogenic. Important genera in this family include Escherichia, Salmonella, Shigella, Klebsiella, Citrobacter, and Proteus. Many biochemical tests are involved in the identification and differentiation of members of this family.
A series of four tests known collectively by the acronym IMViC is used to differentiate between Escherichia, Enterobacter, and other genera. The indole test is positive for organisms that have tryptophanase. The methyl red and Voges-Proskauer tests examine differences in glucose fermentation among species. The citrate test is positive for species that are able to use citrate as a sole source of carbon.
Other tests are available to differentiate Enterobacteriaceae. The urease, phenylalanine deaminase, and decarboxylase tests detect differences in enzyme activities among species. The Kliger’s iron agar test differentiates Enterobacteriaceae based on how the species ferment lactose and glucose. Hippurate hydrolysis is a positive test for Campylobacter.
Haemophilus is another gram-negative rod. The X and V factors test can differentiate this species (which requires the factors) from other Haemophilus species.
Staphylococci and streptococci are gram-positive cocci usually grouped in clusters and chains. The catalase test is valuable for distinguishing between the genera; Staphylococcus is positive and Streptococcus is negative. The coagulase test is positive for Staphylococcusaureus. Lysostaphin is an enzyme that specifically breaks down Staphylococcus strains. The bile solubility and optochin disk tests are positive for Streptococcus pneumonia but negative for other beta hemolytic streptococci. The litmus milk test differentiates among streptococci based on lactose fermentation. Finally, the CAMP factor test is positive for group B Streptococcus.
The Neisseria genus is a gram-negative diplococci with a characteristic doughnut shape. Neisseria can cause gonorrhea and meningitis, so several tests have been developed to differentiate the species. The nitrate reduction test is negative for N. gonorrhea but positive for closely related species. The DNase test is also negative for N. gonorrhea. The acid detection test detects Neisseria species that metabolize carbohydrates by oxidative pathway rather than by the more common fermentative pathway. The oxidase test is positive for Neisseria and Moraxella and can also help to differentiate among many related species in conjunction with other tests. The carbohydrate utilization test distinguishes between N. gonorrhea that ferments glucose only and N. meningitis that ferments both glucose and maltose.
Clinical biochemistry laboratories are responsible for most of the tests performed on samples sent to diagnostic laboratories by clinicians. The results of biochemical tests are used by medical staff for diagnosis in approximately 70 percent of all cases. Compared with other medical tests, biochemical tests are generally easier to perform but are relatively expensive. They can help prevent misdiagnosis by distinguishing between closely related species.
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