Sloppy technique blamed in lab contamination
Sloppy technique blamed in lab contamination
Replace those stoppers as you work, expert says
A recent study shows isolates in several New Jersey laboratories had been unwittingly contaminated by a virulent strain of TB.1 The strain, Mycobacterium tuberculosis H37Ra, often is used as a control in labs to make sure processing chemicals aren’t too harsh. It’s also a favorite in susceptibility testing, where it’s used to make certain that a susceptible strain of TB in fact cannot grow in drug-containing media designed to kill it.
Along with being safe, H37Ra is a fastidious critter that’s actually a bit harder to culture than its disease-causing cousin. So how did the slip-ups occur?
"Often that points to sloppy technique," says Jack Crawford, PhD, head of the mycobacteriology lab at the Centers for Disease Control and Prevention in Atlanta. "In most cases, it’s simply a matter of having more than one tube open at a time."
Here’s how a lab can get into trouble, adds Crawford: Frequently, a lab tech will dispense media or reagents into specimen tubes using a bottle perched on top of a safety cabinet; the bottle is usually equipped with a rubber tube with a pinch clamp on the end.
The fastest way to fill a rack of test tubes is to go down the row, assembly-line-style, dispensing 5 ml into each tube. The trouble is that the technique can result in splash-back. "Sometimes there are splashes onto the tip of the dispenser, or into another tube," Crawford says.
A better way is to dispense reagents into tubes is individually. "Once the media or reagents have been dispensed, you put the cap back on the tube," he explains. "You never use the same batch of reagents for a whole series of tubes, and you don’t take the lids off a series of tubes at the same time."
As a backup precaution, labs should also be on the lookout for a single positive specimen in a series where the others are negative, Crawford adds. "Especially if the positive specimen has only a small number of colonies, or if the colonies take a long time to grow, that should be a red flag," he says. "That doesn’t mean [the positive result] is not valid — just that you should let someone know that this is a questionable result, and that maybe they should take a long, hard look at that patient."
Reference
1. Centers for Disease Control and Prevention. Misdiagnosis of tuberculosis resulting from laboratory cross-contamination of Mycobacterium tuberculosis cultures — New Jersey, 1998. MMWR 2000; 49:413-416.
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