Some acronyms that could help you read this post:
AFB = acid fast bacillus
MGIT = mycobacteria growth indicator tubes
And this post is a continuation of this one.
Remember the stats I showed you a few posts ago? Yeah, looks like things went well last month. The decontamination rate now makes sense. To recap, going below 2% means the lab might have over-decontaminated patient samples. In other words, there could be false negatives. On the opposite end, going over 8% means there could be more false positive cultures.
As I have mentioned before, this number has more significance to the lab than actual patients. Having false anything cost more time and effort on the lab's part to come to a definitive conclusion. Remember, an AFB culture can take up to 6 weeks to resolve.
At my work, we use BD BACTEC™ MGIT™ automated mycobacterial detection system
Source.
When the machine detects a positive
The "positive" tubes need a gram stain and an AFB stain each. They would tell us the type of growth that are happening inside the tubes. Sometimes, cellular debris can trigger a positive warning.
Interpretations
Depending on the stain results, different actions would follow. I'll use the equal sign to denote "negative" because it's easier to read.
AFB+/gram stain=
This is a true positive result. This means that the isolation and culture was a success. At my work, we ship these to a reference lab for identification and susceptibilities. Most hospitals do not have the capability to perform the mentioned tasks.
AFB+/gram stain+
This is where the lab would have to do more work. It is in the lab's best interest to send off a pure culture for further workup. There are a few things to perform here.
The MGIT would require another decontamination. The protocol now demands the use of NaOH and its XPR-Plus neutralizing buffer.
Given there is mycobacteria in the tube, we can use Mitchison 7H11 selective agar. As you might expect, this particular agar only allows AFBs to grow.
The XPR and 7H11
Some may wonder, why not use the selective agar in the first place? The answer to that question is simple: there may not be enough AFB to show up on the agar without culturing. It's a matter of sensitivity.
The concept is like using thioglycolate broth to enrich small number of organisms. The broth is a differential media, but the most common use of them in my department is for enrichment.
From this point, we would wait for either the new MGIT or 7H11 to show growth. If the MGIT comes up mixed again, we would resort to the 7H11 and the LJ slant. Sometimes, we do decontaminate the MGIT another time, but that depends on how much other bacteria are present.
AFB=/gram stain+
A decontamination run is in order. If this is the second time it showed up positive, it's time to toss it. We would observe the LJ slant until we can determine the culture has been overrun by other bacteria. That's what we would tell the clinicians in that scenario.
It's not uncommon for other bacteria to fester in an AFB culture. This is especially true if the patient is battling other infections. The clinicians would have to decide whether they want to pursue another order.
AFB=/gram stain=
In this case, you would still perform a decontamination. The number of bacteria could be low, or there are enough debris in the culture to trigger a positive warning.
To be on the safe side, we would still add 7H11 in case something grows.
In the end
This type of culture takes the longest time to process and culture. On average, I spend 3-5 hours a night of my 10-hour shift dealing with them.
Of course, this is a brief rundown of how a clinical lab deal with a potential mycobacteria infection. I have left out a lot of the details, but I didn't think anyone would want to read about the nitty-gritty stuff. I try to keep things simple, but it most likely over most people's heads anyways.