TB diagnosis special report: why smear microscopy needs to be improved

Theo Smart
Published: 23 March 2007

“It’s clear that in the presence of HIV, smear microscopy [the lab test for active TB] is severely compromised and that we need new tools. But in the meantime perhaps there are ways we can modify the smear approach or modify the sputum processing in order to get the best out of smear microscopy until these new tools come on the scene,” said Dr Andy Ramsey from the Special Programme for Research and Training in Tropical Diseases (TDR) and the STOP TB Working Group on New Diagnostics, speaking at a symposium held by the Foundation for Innovative New Diagnostics (FIND)] at the 37th World Union Conference on Lung Health, held in November 2006 in Paris.

Even though smear microscopy has been much maligned due to its poor sensitivity — in practice, it may miss up to half of the cases of active pulmonary TB (and even more of the cases in children and HIV-infected people) — the lab test remains the mainstay for TB diagnosis in resource-limited settings. Despite a substantial (though still not adequate) increase in TB diagnostics research, there are, as of yet, no new lab tests that are simple or sensitive enough to be replace smear microscopy in the under-resourced peripheral laboratories in the developing world.

But over the course of the TB conference, a number of strategies and or initiatives to make better use of smear microscopy were described that could significantly increase successful diagnoses and decrease costs to both public health systems and patients in resource-limited settings.

“At the peripheral level, FIND is focused on incremental improvement in smear microscopy,” said Dr. Vinand Nantulya. He said that a few of these strategies are going into large scale demonstration phase this coming year and could be available for integration in the programmatic level by 2008/2009.

While these ‘incremental improvements’ will not completely make up for the shortcomings of the test, in countries with a high burden of TB, even small improvements to the test’s availability, cost and performance could save thousands of lives.

The strategies include:

  • Increasing access to smear microscopy in high burden countries (including an effort to distribute “smear microscopy kits”)
  • Reducing the number of specimens and clinic visits required of the person with suspected TB in order to get a diagnosis
  • Improving diagnostic yield by improving sputum processing (such as adding a step of bleach digestion)
  • Introducing low cost fluorescence microscopy — possibly using inexpensive and long-lasting LEDs as a light source.

Although many people with HIV and TB may have smear-negative TB (which cannot be detected by smear microscopy), improving the performance of this test at the peripheral level should still be of benefit to them, because smear microscopy is still the first step and first opportunity for a diagnosis. A substantial proportion — but by no means all —of people with HIV may actually have TB that could be detected with better equipment, specimen processing and quality control; and a diagnosis on the basis of smear microscopy will lead more rapidly to a life-saving course of anti-TB drugs.

Furthermore, smear microscopy is key to the diagnosis of some forms of extrapulmonary TB, using biological material from the suspected site of infection — such as fine needle biopsy of lymph nodes, in the case of TB lymphadenitis.

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