Gram Stain Quality control is one of the most neglected practices in less organised laboratories.
Gram Stain is the simplest and most important test in medical microbiology laboratories.
This was developed by a man called Hans Christian Gram in 1884.
One thing a good medical laboratory does is to have a quality control set up for every test they perform, including Gram stain.
Table of Contents
Gram stain quality control set-up.
There are two kinds of quality controls concerning Gram stain procedure.
They are:
1 Procedural
2 Precautionary
Procedural quality control in Gram Stain
This is a standard quality control procedure that must be established in a microbiology laboratory.
This is done by buying reference bacteria strains(standard strains). The reference bacteria must be both Gram-positive and Gram-negative.
Examples of bacteria strains you can use are:
Escherichia coli (ATCC 25922) for Gram-negative Quality control
Staphylococcus aureus (ATCC 25923) for Gram-positive Quality control
ATCC stands for American Type Culture Collection. This organisation has banks of thousands of bacteria species used in microbiology labs worldwide.
The ATCC 25922 and ATCC 25923 are cultured on appropriate culture media, such as Blood agar, for 18-24 hours. Then, make slides of each bacteria.
You can make 20 slides of each and label each as QC. For example, the slides for ATCC 25923 can be labelled “Gram-positive QC”, and the other, ATCC 25922, “Gram-negative QC.”
You now have 20 Gram-positive quality control slides and 20 Gram-negative quality control slides.
You will store them in a cool, dry box in the lab.
Before you perform Gram stain on patient samples every morning or once a week, you will first pick the QC slides and perform Gram stains on both Gram negative and Gram positive slides.
View the stained QC slides under the microscope. If the Gram-negative QC slide shows Gram-negative and the Gram-positive slide shows Gram position reaction, then your daily or weekly Gram stain quality control has passed.
Record the result in your record book or electronically if that is what is obtainable in your lab.
Record also the date and time of the QC test and sign. That is all.
Without this documentation, you can’t prove you have done any quality control.
Remember to keep or discard the stained QC slides, depending on your laboratory policies.
However, if you can’t buy the standard strains, you can use the bacteria species you identified in your lab.
You can pick one Gram-positive and one Gram-negative bacteria you isolated and prepare the slides as described above.
Suppose an internal or external auditor walks into the lab and requests to see the Gram stain quality control materials. In that case, you can proudly show him or her the slides and documentation to prove how dedicated you are to doing the right thing.
This is for procedural quality control, so we examine the next step.
Precautionary quality control in Gram Stain
This is as important as procedural quality control. Here, the quality control practice is loose rather than rigid.
This includes buying the right or quality Gram stain reagents. If you purchase nonstandard reagents, you will get funny Gram stain results.
It is simple garbage in, garbage out!!
Precautionary quality control in Gram stain also involves correctly timing the Gram stain procedure, especially the stage of decolonization with alcohol.
If not done correctly, decolourisation timing can make Gram-positive bacteria look Gram-negative. This is called over decolorisation.
There are different grades of alcohol. The higher the grade, the less time it is allowed to be in contact with the bacteria on the slide before washing it off with water.
If the grade of alcohol you are using is low, the contact time has to be more. That is why you can see different contact times in the SOP of different hospitals or labs.
Another factor influencing contact time is the thickness of the Gram stain smear you made. The thicker, the more time is allowed for alcohol to act, like sputum. The thinner the smear, like csf samples, the lesser contact time.
These are to ensure correct staining and avoid over or under-decolorization.
You also need to use very clean water for Gram stain procedures. Dirty water is a no-no for anything in a medical lab, including Gram stain procedures.
Distilled water will not harm you if you use it for your Gram stain tests. Will it?
You can read more about Gram stain here
