sensitivity and problems with Lab reports

[Guest guest]

Hi,

this article emphasizes one aspect of the antibiogramms in the Lab

report. If the report says, that a bug is sensitive to an antibiotic

in the petri-dish, it doesn't automatically mean that it works in the

body of a pwcf too.

But it can also happen the other way round. We have mentioned this in

the past about TOBI. Even id the report says, that pseudomonas is

resistent to tobramycin TOBI still may work, because the lab usually

works with concentrations similar to an IV and if you inhale TOBI the

concentration in the lung will be much higher than with an IV.

And finally there is still the chance of synergetic testing. Two or

three antibiotics that can't kill a bacteria alone sometimes can do

it if they are given together.

Peace

Torsten

Antibiotic sensitivity testing may not reflect clinical need, research

claims

BODY:

The standard laboratory test used to help choose the

most appropriate antibiotic for treating bacterial infection might not

always be suitable, for patients with severe infection, researchers

from

Queen's University Belfast reported this week at the British

Pharmaceutical Conference.

Antibiotic susceptibility tests are used routinely to

determine which antibiotics are effective against the infecting

organism. British guidelines specify the conditions for the tests and

the number of bacteria [usually 5x105 colony forming units/ml] to be

used in the test sample.

The researchers say that these test conditions do not

always reflect the clinical situation. For example, cystic fibrosis

[CF]

patients with lung infection have been shown to have a much higher

number of bacteria at the site of infection [8x108 cfu/ml].

They investigated whether this higher inoculum size

affected the results of susceptibility testing. The antibiotic

sensitivity of two bacteria which frequently cause CF lung infection

[Pseudomonas aeruginosa and Burkholderia cepacia] was tested, using

both

a standard and a high inoculum size. For one antibiotic, piperacillin,

the minimum inhibitory concentration [MIC, the lowest concentration

that

inhibits bacterial growth] was up to 60 times higher for the higher

inoculum than the standard inoculum. MIC increases with the other

antibiotics were less marked.

Dr Tunney, from QUB, explained the practical

significance of this finding: " While the standard test might suggest

that a particular antibiotic is going to be clinically effective, in

practice the drug might not work because of the higher concentration

of

bacteria present. "

He said that they also noted potential problems with

the standard test when patients had a mixed infection [more than one

infecting organism]. " With the standard test, antibiotic sensitivity

is

measured separately for each organism. We tested two strains of

bacteria

separately, and then together, and found that in some cases the MIC

for

an antibiotic increased when the two were tested together. "

The researchers concluded: " Our study has shown that

when determining antibiotic sensitivity using in vitro tests it is

important that conditions reflect the in vivo situation. In certain

circumstances, it may therefore be appropriate to use a higher

inoculum

and when infection is caused by more than one strain of bacteria it

may

be necessary to test efficacy of the antibiotics against the infecting

bacteria in mixed culture. "