Microbiologia Laboratorio

College of Physicians & Surgeons of Saskatchewan
Laboratory Quality Assurance Program

Procedures/Guidelines
for the Microbiology
Laboratory

January 2010

Table of Contents

Microbiology Procedures/Guidelines – 2010 Edition

Page 1

QUALITY CONTROL IN MICROBIOLOGY LABORATORIES
Microbiology laboratories shall use quality control procedures to ensure the accuracy,
reliabilityand reproducibility of the various tests used in the isolation, identification and
antimicrobial susceptibility testing of microorganisms, and in the performance of
serological testing. The extent of quality control testing done will be determined by the
scope of clinical testing performed in each laboratory.
All laboratories performing microbiology shall have appropriate internal qualitycontrol
procedures using CLSI guidelines for antibiotic susceptibility testing and quality control
of media.
a)

The most recent update of the following CLSI documents should be followed:
(i)

M22 Quality Assurance for Commercially Prepared Microbiological
Culture Media

(ii)

M100 Performance Standards for Antimicrobial Susceptibility Testing

(iii)

M2

(iv)

M7
Methods forDilution Antimicrobial Susceptibility Tests for
Bacteria that Grow Aerobically

(v)

M11 Methods for Antimicrobial Susceptibility Testing of Anaerobic
Bacteria

Performance Standards for Antimicrobial Disk Susceptibility Tests

b)

Identification panels should be quality controlled according to the manufacturer's
recommendations.

c)

ATCC organisms shall be used for proper qualitycontrol of media and of
antimicrobial susceptibility testing.
CLSI: Clinical and Laboratory Standards Institute
ATCC: American Type Culture Collection

All laboratories performing microbiology shall demonstrate and document their efforts to
meet the standards established by CLSI guidelines and quality control using appropriate
organisms.

Microbiology Procedures/Guidelines – 2010 EditionPage 2

GUIDELINE FOR QUANTITATIVE INTERPRETATION OF GRAM STAINS
Background:
Microscopic examination remains the initial diagnostic test in the processing of
specimens in the clinical microbiology laboratory. The timely report of a Gram stain
result gives the physician important information about the presence and cause of
infection. The Gram stain has a broad staining spectrum andclassifies bacteria as either
Gram-positive or Gram-negative. Despite the clinical importance of the Gram stain,
there are few available standards for the reading and interpretation of this test. The
assignment of semi-quantitative and quantitative values to the number of cells and
bacteria seen is clearly arbitrary, since published criteria for general use vary
dramatically(1-5). Althoughlaboratories may report only semi-quantitative Gram stain
criteria, it is clinically important to have a standardized schema for assigning quantities of
cells and bacteria to individual semi-quantitative scores. Since several clinical studies
have shown that the presence of moderate to heavy amounts of pus and/or the presence of
bacteria on Gram stains correlates with the presence of infection(6-8),information reported
from Gram stains of specimens needs to be accurate.
Principle:
To review current practice in Canada and evidence from the literature to establish a
current best practice approach to the quantitative interpretation of Gram stains for all
types of clinical specimens.
Recommendation:
Although there are several published Gram stain reporting criteria, the Canadian
Coalitionfor Quality Laboratory Medicine recommends that:
Laboratories use:
o One set of interpretive criteria for all types of clinical specimens except for
sputa and vaginal smears for bacterial vaginosis.
o Separate published specimen rejection criteria for sputa.
o Standard criteria for diagnosis of bacterial vaginosis(9, 10).
Laboratories specify the criteria used when reporting Gram stain...