Evaluation Of Disinfectants

A number of laboratory tests have been developed to evaluate the efficacy of disinfectants (2). The Rideal-Walker


Hypoclorite Iodophors ammoniums

Biocidal spectrum

Gram-positive bacteria

Gram-negative bacteria

Bacterial spores



Viruses pH effectiveness




Stability to

Organic residues

Hard water


Skin irritant

Residual activity

High temperature stability

Use solution stability

Maximum use concentration by FDA°

25 mg/16

200 mg/16

Note: Relative properties: + +, definite; + moderate; ±, variable, —, very low to none.

"According to 21CFR, 178:1010; no rinsing after application.

6mg/L = milligrams per liter or ppm.

and Chick-Martin methods of determining the phenol coefficient with Salmonella typhi were developed in the early 1900s. Although the Chick-Martin test includes an organic soil, the phenol coefficient tests are artificial in concept, have poor reproducibility and phenol is an unreliable control disinfectant (1). The improved Kelsey-Sykes capacity test attempted to resolve the deficiencies of the phenol tests by using at least four organisms in preliminary screening tests, the most resistant organism being selected for further testing. The disinfectants are prepared in a standardized hard water, with or without a standard soil. Recovery broths are prepared with a neutralizer against the disinfectant. This test also has its shortcomings, but it is used as the official test for disinfectants in the UK (21).

Several tests have been designed to simulate in-use conditions by preparing an air-dried film of microorganisms on an appropriate surface, such as stainless steel, with or without an organic soil. The Lisboa test was developed for testing sanitizers for dairy equipment. Stainless steel tubes are contaminated and disinfected using standardized procedures and neutralized to inactivate any residual disinfectant; the number of surviving organisms is then determined (1). Various modifications of this in-use testing have been developed (2,22). Methods of the Association of Official Analytical Chemists (AOAC) include use-dilution carrier techniques with Salmonella choleraesuis, Staphylococcus aureus, or Pseudomonas aeruginosa contaminated onto polished stainless-steel cylinders. Tests for fungicidal and sporicidal activity are also specified (23).

Tests for evaluating skin germicides under in-use conditions have been even more difficult to standardize (24), and there are no official methods comparable to the AOAC use-dilution technique for evaluating disinfectants for use on inanimate surfaces. Multiple-basin techniques measure the rate of mechanical removal of microorganisms from the skin. Testing of skin disinfectants must take into account the transient and residual microflora of the skin, the time of exposure, methods of sampling the skin after washing, and methods of contaminating skin with a transient microflora appropriate to the use environment (25). Depending on the germicidal agent used, it may be necessary to neutralize the rinse solution. With skin disinfectants it is appropriate to test for a residual (or substantive) effect, ie, improved results with repetitive use of the same agent compared with nongermicidal soaps. Occlusion and expanded flora tests (in which agents are applied topically to the skin, covered with a plastic film, and sealed with impermeable plastic tape) represent alternative approaches to testing skin disinfectants (26).

Comparisons of antimicrobial soaps by various standardized techniques have given results that question the in-use efficacy of most products tested. Studies have indicated that under the specific test conditions, only an io-dophor product (0.75%) and 4% chlorhexidine gluconate significantly reduced the number of microorganisms released from hands (25,27,28). Other products, including intermediate strength iodophor products, 2% chlorhexidine, Irgasan DP 300, para-chloro-meta-xylenol (PCMX), tri-chlorocarbanilide or tribromosalicylanilide, under in-use conditions of testing were no better than a nongermicidal soap.

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