Yes, TB especially MDR-TB diagnosis conventionally does take 6 to 8 weeks.
Standard practice for TB diagnosis
- Do sputum* samples contain acid-fast (TB) bacilli when examined under the microscope (smear microscopy)?
- Does cultured sputum yield TB bacilli?
- Such cultures are then subjected to anti-TB drug sensitivity tests (DST).
TB microbe is extremely slow-growing, taking 3 to 4 weeks to generate a logarithmic culture. Compare this to E. coli which divides every 20 minutes! TB bacilli’s exceptionally slow growth is why TB diagnosis takes 6 to 8 weeks.
* sputum is the mixture of saliva and mucus that comes up from upper respiratory passages in respiratory diseases. Unlike saliva which is the product of salivary glands, sputum includes secretions of trachea and lung bronchi as well.
A newer TB diagnostic method, Xpert MTB/RIF assay takes a matter of hours. It’s an automated real-time nucleic acid amplification technology for rapid and simultaneous detection of tuberculosis and Rifampicin resistance using the GeneXpert instrument (Cepheid, Sunnyvale, CA, USA).
From Reference at end
The WHO is the ultimate authority that periodically convenes expert panels who analyze available data at length and recommend optimal TB diagnostic methods. Thus, in 2013, ‘WHO’s current policies and guidance recommend that Xpert MTB/RIF be used as an initial diagnostic test in individuals suspected of having MDR-TB or HIV-associated TB (strong recommendation, moderate quality of evidence)‘ (From Reference at end).
From Reference at end
Advantages of Xpert MTB/RIF assay for TB and MDR-TB diagnosis
- Xpert MTB/RIF assay is more sensitive than smear microscopy with average sensitivity of 88% and 65%, respectively, from 21 comparative studies with a total of 8880 participants.
- These numbers mean Xpert MTB/RIF assay could detect TB bacilli from sputum in 23% of cases that were missed by the standard sputum microscopy method.
- Costs less than conventional diagnostics for MDR-TB and HIV-TB: Xpert MTB/RIF assay would cost ~US$90 million and ~US$100 million per year globally to diagnose MDR-TB and TB in HIV-positive, respectively.
- GeneXpert set-up cost is much higher than microscope (~US$17500 versus ~US$1500) but much lower than standard culture and drug sensitivity testing, which cost ~US$300,000 for an established lab and ~US$1.4 million for a new lab. The older tests need extensive biosafety equipment and infrastructure such as autoclaves, culture media preparation and consumables.
Drawbacks of Xpert MTB/RIF assay for TB and MDR-TB diagnosis
- Costs more than conventional diagnostics for routine TB: While total Xpert MTB/RIF assay costs are less than 10% of current TB funding programs in Europe, Brazil and South Africa, they are much higher or could be prohibitive in poorer countries. Xpert MTB/RIF assay would cost ~US$470 million per year globally to test everyone who showed signs and symptoms of TB.
- Xpert MTB/RIF assay only detects Rifampicin resistance so countries with documented XDR-TB (extensively drug-resistant TB) also need conventional culture diagnosis and resistance assessment to second-line antibiotics.
- Cartridges are bulky so labs need to have large storage space.
- A single 4-module GeneXpert instrument can only process 16 to 20 specimens per day. TB-endemic regions would need at least 4 such instruments.
- GeneXpert instrument can be operated at maximum temperature of 30oC. Xpert MTB/RIF assay cartridges and specimen reagent need to be stored at 2 to 28oC. Much lower than ambient temperatures in many TB-endemic areas so rooms housing instrument(s) and supplies need to be air-conditioned.
- Xpert MTB/RIF cartridges have a maximum shelf-life of 12 months. TB-endemic areas could take longer to get them cleared through customs! Inventory management would be quite challenging to say the least.
- GeneXpert modules require annual calibration that can only be performed by an authorized service provider. Remote calibration is a possibility but if it failed, module needs to be exchanged.
- The WHO can only make recommendations. Each individual country’s Ministry of Health decides to implement or not the Xpert MTB/RIF assay within the context of their National TB management plan. Work-in-progress.