Microbiology Lab Organization

The microbiology lab is a division of the department of laboratory medicine. The medical microbiologist is responsible for overall laboratory policy and he or she answers to the head of the department of laboratory medicine. In labs without medical microbiologists a general pathologist or a Ph.D. microbiologist (hopefully with special training in clinical microbiology) may oversee the lab.

Technologists, stratified according to experience and training perform the day-to-day processing of specimens and a laboratory manager oversees them. Microbiology technologists, in general, are an excellent resource for physicians and they handle many routine queries. Any issue they feel they can’t handle is referred on to the medical microbiologist.

The lab serving your region will have specific hours and likely has a call-back system for important after-hours specimens. It is very important for the ordering physician to understand the regular hours and overall staffing of the lab in order to use it most efficiently. For example, if working at night and feel that a patient in ICU needs a sputum culture but not an acute change of therapy it would be more appropriate to collect it first thing the following morning rather than at night and have it sit on the unit for several hours.

Ordering, Labeling and Requisitioning of Specimens

There is an oft quoted dictum of laboratory service: Garbage in = Garbage out. It is the ordering physicians responsibility to ensure that the proper specimen is selected, requisitioned, obtained, placed in the proper container, transported to the laboratory and left in the proper place if after hours. This is often done very poorly either do to lack of care, lack of information or lack of structure. Often one simple phone call to the lab is the difference between diagnosis and disaster.

The following sections will address the individual steps of this “pre – lab” phase of testing.

Ordering of Microbiologic Tests

Before a physician orders a microbiologic test consideration to the following questions is essential:

Is this test indicated?

Microbiologic investigation is very expensive and labour intensive. Consider whether the outcome of the test will directly influence clinical decision-making. Example: A woman presents with typical symptoms of cystitis has many WBCs detected in a dipstick urinalysis of her urine. You elect to treat her empirically with nitrofurantoin. Culture of her urine is unlikely to change her clinical course and should not be ordered. If she fails therapy, which is unlikely, culture can and should be done at that time.

Should I collect the specimen myself?

Yes, whenever possible. A common problem is that physicians leave orders for swabs to be taken and assume nurses know how to take them. They don’t, and shouldn’t be expected to. Anything that is worth sampling is worth being sampled by the physician and if you don’t take the specimen you may have great difficulty interpreting the result.

Labeling of Specimens

A specimen that arrives in the lab unlabeled is not processed! It is imperative that attention is paid to this crucial part of sample acquisition. Specimens should be labeled after they are obtained. Pre-labeling of specimen containers can have disastrous consequences. The person who obtains the specimen should label the specimen or at the very least ensure that it has been done properly. The procedure is not complete until the specimen is in a properly labeled container. The label should include:

  1. Name in full
  2. Health care number
  3. Date of birth
  4. Date and Time sample was taken. (Very often time is not indicated)

Completing Requisition Forms

The responsibility for filling requisitions is very often left to unit clerks who know very little about the patients. This is an unacceptable circumstance. It takes very little time to complete a requisition and should be the responsibility of the physician, in particular for more complex specimens. The more information that is received in the lab about the patient and the specimen, the more help you will receive from the lab.

The important components of a requisition include:

  • Name in full
  • Health care number
  • Date of Birth
  • Date and time of specimen collection
  • Ordering physician
  • Person collecting specimen
  • Clinical indication for test and provisional diagnosis
  • Type of specimen including source and method of collection
  • Tests requested
  • Indication of immune status (e.g. Neutropenic, AIDS)
  • Antibiotics in use
  • Any special instructions for the lab

Lab Techniques

Semi quantitative culturing

Swabs are inoculated onto the surface of agar plates in an area approximately the size of a quarter taking care to have all portions of the swab tip contact the plate. The inoculum is then streaked over the surface of the plate in 4 quadrants:

Amounts of bacteria are graded according to their presence in the 4 quadrants as follows:

  • Heavy (++++) growth all quadrants
  • Moderate (+++) growth 2nd to 3rd quadrants
  • Light (++) growth only in 1st quadrant but >10 colonies
  • Very light (+) growth < 10 colonies