Making organism identification cost-effective

Medical Laboratory Observer, March, 1986 by Michael T. Kelly

In addition, when the rule-out anaerobes option is chosen, the lab can identify Bacteroides fragilis using fluorescent antibody reagents and Clostridium perfingens, which can be identified from colony morphology and hemolysis on the primary growth medium. In practice, the rule-out anaerobes option is the most often selected, followed by genus level identification. Species level identification is rarely requested.

Nonfermenters. P. aeruginosa, accounting for at least 80 per cent of nonfermenter isolates in most laboratories, can be identified with 98 per cent accuracy through colony morphology and the spot oxidase test. For more extensive testing on the other 10 to 20 per cent of nonfermenter isolates, classic biochemical tests are recommended because of their accuracy and low cost. More expensive commercial systems have less than 90 per cent accuracy.

Complete identification of nonfermenters is often unnecessary because they occur with multiple organisms in wound and sputum specimens and are therefore of questionable significance.

Gram-positive bacilli. Since the vast majority of gram-positive bacilli isolates are contaminants and not associated with disease, these organisms require only a minimal workup. Evaluation should be adequate enough to detect significant organisms, such as Listeria, Corynebacterium JK, C. diphtheriae, B. anthracis, etc. They should be ruled out when isolated from normally sterile body fluids and when clinically suspected from other sites.

If, for example, a diphtheroid is isolated from a spinal fluid specimen, it would be important to rule out Listeria with a hanging drop motility test. If there is motility, additional testing should be done to identify a potential Listeria. For most gram-positive bacilli, such tests as Gram stain, catalase, hanging drop motility, colony morphology, and susceptibility pattern will suffice.

The extent of organism identification also depends on the type of specimen yielding the organism. Figure II shows the extent of workup for isolates from blood and body fluids, sputum, throat, urine, and wound specimens.

Organisms present in specimens such as blood or normally sterile body fluids merit extensive identification because they are likely to be associated with infections. All blood and body fluid isolates, therefore, should be identified.

With sputum, the degree of testing depends upon the quantity of organisms and the cytological quality of the specimen. Gramnegative bacilli, staphylococci, beta streptococci, and possible pneumococci should be worked up if the quantity is 2 (10 or more colonies in the primary inoculation area). When more than two potential pathogens are present in a sputum culture, it is reasonable to identify the predominant organism and report morphological types for the others.

With throat specimens, labs should consider the methods now available for directly extracting the antigen of group A streptococci from throat swabs without culturing.3 These tests compare reasonably well in accuracy and cost with traditional throat cultures, and they are much faster.