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Technology’s an overachiever, experts say
Since 1996, seven sites across the country have been doing DNA fingerprinting on all their TB isolates. Now, it’s time to evaluate the results, says a TB laboratory expert at the Centers for Disease Control and Prevention.
"We wanted to see whether doing this would benefit TB control programs, to see whether it was practical and cost-effective, and to look at how much work it is," says Jack Crawford, PhD, chief of the immunology and molecular pathogenesis section of the Division of AIDS, STDs, and TB Laboratory Research Division at the CDC in Atlanta.
What Crawford’s found seems to be a combination of good and not-so-good news.
On one hand, it’s clear that TB control programs see tangible benefits from strain typing, because it lets them zero in on ongoing transmission that otherwise might have been missed. Programs "like [using the technology] a lot," he adds.
At the same time, the technology is costly and entails a lot of work. Arguably, it provides more information than programs actually need, he adds. That’s why he and other researchers are looking at whether there may be cheaper, simpler ways to accomplish the same thing — maybe by using less costly, simpler methods to pre-screen isolates before deciding whether to go ahead with strain typing.
A little too much detail?
"The trouble with DNA fingerprinting is that it’s very specific, as well as fairly specialized," Crawford notes. After strain-typing about 16% of all TB isolates in the country, the seven-site project has turned up a whopping 6,000 fingerprints out of 12,000 isolates. "That’s a little more specific than you really need," he adds. Plus, the technique is not one that every lab can implement.
What Crawford would like to try out is a handful of simpler, cheaper screening tools, including two methods using polymerase chain reaction (PCR). One PCR-based method involves using a hybridization assay; the other searches for the same sequence of alleles repeated end-to-end a variable number of times.
Another possible screening tool is spogliotyping. This is another kind of hybridization assay in which the aim is to determine the presence or absence of a particular sequence expressed in a digital form.
It has yet to be determined when and where the new technologies will get a workout, Crawford notes. "We haven’t made up our minds yet about how we’ll implement this — probably not this year, but maybe next year."
DNA fingerprinting has been used since the early 1990s in outbreak investigations. Landmark studies of the technique by San Francisco TB researchers encouraged other programs to begin using it on an ongoing basis as a way to ferret out ongoing transmission. Sites for the CDC project include Massachusetts, New Jersey, Michigan, Maryland, parts of Texas, and California.