The reduction factor (RF) - An exampel of wishful thinking?


In the Quality Control (QC) program for cleaning and disinfection of flexible endoscopes at  Copenhagen hospitals we had detected the same micro organisms in samples from flexible endoscopes (obtained immediately before an endoscopy) and in blood cultures from  patients after endoscopy.

 "The classic idea that a disinfection is carried out successfully if the procedure has succeeded in reducing the germs by 3-5 log-stages does not apply here" (Hyg. Med 1994:19;75-83)

The question is: Are the condition for using the reduction factor fulfilled?

An example

A FE was contaminated with a test germ in a laboratory evaluation of a WD. Samples are obtained before an after the reprocessing in the WD. The sampling results   were: mean log CFUpredisinf.=6 and mean log CFUpostdisinf. = 2.

In a rapport was concluded: The test germ is reduced by 4 log-stages and mean log RF = 4.

Figure1: A constructed example of an evaluation of a washer-disinfector (b=the slope)

The mean log RF is defined as 

In everyday life we use

Using mean log RF we presuppose, that the relation between log CFUpredisinf. and log CFUpostdisinf. is a stright line with a slope=1.


Is the relation between log CFUpredisinf. and log CFUpostdisinf.

  1. a stright line with a slope = 1?
  2. a stright line with a slope = 0?
  3. an other relation than above?

We call for a documentation!


Cold-chemical disinfection


The definitions of clean, critical and high-risk FE are showed at table 1. 10 drops of 20µl (total amount 0.2 ml) were spotted on 5% blood agar.

Table 1: 60 samples from the water channel of two Olympus flexible gastroscopes after manual cleaning and disinfection in a washer disinfector with 2% alkaline activated glutaraldehyde for 10 minutes.

Proliferation of bacteria over night in the gastroscopes


Sampling after

 Clean endoscopes

0-4 CFU per 0.2 ml

 Critical endoscopes

5-49 CFU per 0.2 ml

 High-risk endoscopes

>=50 CFU per 0.2 ml




Manual cleaning


 40 (95 %)


2 (5 %)

















Manual cleaning

















There were no correlation between CFU before and after disinfection, and conditions for using the RF are not fulfilled.

QC results and conclusions

Samples from the water channels of FE immediately before about 3.000 endoscopies has showing that cleaning and cold-chemical disinfection of FE are satisfactory

Laboratory evaluation of WDs

Using our protocol for laboratory evaluation of WDs the disinfection of FEs fail if WD doesn’t prevent proliferation of the test strains during storage over night. Test day 4 and 5 are the critical days. The results from day 4 and 5 are weary like results from a in use test.


Nosocomial Infection from contaminated FE