What are biochemical tests?

Definition

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.

Enterobacteriaceae

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

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.

Neisseria Species

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.

Impact

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.

Bibliography

Forbes, Betty A., Daniel F. Sahm, and Alice S. Weissfeld. Bailey and Scott’s Diagnostic Microbiology. 12th ed. St. Louis, Mo.: Mosby/Elsevier, 2007.

MacFadden, Jean F. Biochemical Tests for Identification of Medical Bacteria. 3d ed. Philadelphia: Lippincott Williams & Wilkins, 2000.

Mandell, Gerald L., John E. Bennett, and Raphael Dolin, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. New York: Churchill Livingstone/Elsevier, 2010.

Murray, Patrick R., Ken S. Rosenthal, and Michael A. Pfaller. Medical Microbiology. 6th ed. Philadelphia: Mosby/Elsevier, 2009.

Murray, Robert K., et al. Harper’s Illustrated Biochemistry. 27th ed. Stamford, Conn.: Appleton & Lange, 2006.

Pagana, Kathleen Deska, and Timothy J. Pagana. Mosby’s Diagnostic and Laboratory Test Reference. 9th ed. St. Louis, Mo.: Mosby/Elsevier, 2009.

Truant, Allan L. Manual of Commercial Methods in Clinical Microbiology. Washington, D.C.: ASM Press, 2002.

Volk, Wesley A., et al. Essentials of Medical Microbiology. Philadelphia: Lippincott Williams & Wilkins, 1995.

Winn, Washington C., Jr., et al. Koneman’s Color Atlas and Textbook of Diagnostic Microbiology. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2006.