Diagnostic Laboratory Section

Interpretation of result

READING AND INTERPRETATION OF RESULTS

• The zone of inhibition is defined as complete absence of any visible growth of bacteria around the disc.
• After incubation, the zone of inhibition is read with the help of scale or caliper.
• The diameter of the zone is read from the back of the plate as it is clear in the case of MH plate and over the surface of the agar in the case of blood agar plates (for Streptococci).
• The transmitted light can be used to see any faint growth of colonies in the margin of the zone of inhibition.
  • Sensitive (Click here to view image)
    • If the zone of inhibition is equal to or above the prescribed limit for the respective antibiotic then the concerned antibiotic can be declared as sensitive.
    • The infection may respond to the treatment at the normal dose level.
  • Intermediate
    • Here the result (zone of inhibition) is equivocal. If the infection could not be controlled, with the prescribed antibiotic then the test should be repeated.
  • Resistant
    • If the zone of inhition falls equal to or below the prescribed limit for resistant category then the concerned drug is said as resistant and such a drug is not effective in controlling the infection.

Consideration during the interpretation

• The zone of inhibition is the area around the disc showing no growth that can be detected by unaided eye.
• However, the following points are considered while reading the results.
  • The large colonies appeared in the zone of inhibition should be subculture and tested again. (If possible should be identified)
  • Faint growth at the edges of the zone may be ignored.
  • In the case of Proteus species, swarming of growth may be observed in to the area of inhibition. If the area of infection is clearly outlined, then the veil of swarming can be neglected.
  • In the case of Sulphonamides and Trimethoprim a light growth in the zone can be ignored and measurements be taken to the margin of heavy growth.
  • The read results (diameter of zone of inhibition for each antibiotic disc usually in mm) are compared with the standards in the chart that is usually given by the antibiotic disc manufacturers.
  • The results are reported as sensitive, intermediate and resistant.