Diagnosis - the complications

The Basic Diagnosis of TB

The same basic principle of diagnosis has been used since Dr Robert Koch identified the TB bacillus 120 years ago.

A process of "gram staining" is used on a sputum sample, after which the bacilli (if they exist) can be identified through a simple microscope. The process is simple and can be done in minutes.

The problem is that this process fails to identify if the bacillus is resistant to any of the existing drugs used in the treatment of TB. Without this knowledge, the clinician is prescribing blindfold, knowing that he may be prescribing treatment that will be not just ineffective, but will also actually potentially stoke an existing drug resistant strain to become that one step more resistant (in other words from MDR-TB to XDR-TB).

Effectively, treating TB without additional diagnostics amounts to a game of epidemiological Russian roulette.

Furthermore, TB patients who are co-infected with TB often fail to test positively for TB using the sputum test, meaning that the entire test can be regarded as being unreliable in the African epidemic.

Diagnosing DR-TB

The key to successful treatment is to determine which drugs (if any) the bacillus may be resistant to. In the developed world this is done automatically on initial diagnosis by taking a blood sample and sending it away for culturing, a process which will identify whether a straightforward six month first line drug regimen will work, or whether a tailor made second line treatment lasting as long as two years will be needed. 

There are two catches to this:

One is that the process takes six weeks.

The other is that these diagnostics require a well-equipped laboratory and are expensive - so expensive, in fact, that they have yet to be made available in almost all of Sub-Saharan Africa despite a known need.

The epidemiological impact of this period of culturing blood

Effectively, this six week period allows any drug resistant strain an opportunity to infect others. In six weeks, an average TB patient is likely to exist more than one other person - possibly more if living conditions are cramped; so, even if the patient is immediately isolated after diagnosis there is a mathematical probability that the epidemic will increase.

New diagnostics

As a result of this, a lot of effort has been expended on developing a faster diagnosis for drug resistance - in fact it's widely accepted that it will not be possible to contain the epidemic of drug resistant disease without one. Recent excitement has been expressed, however, concerning the Xpert diagnostic device, a diagnosis that can be done in hours, not days or weeks. The price (per diagnosis) has also been reduced.

To date, unfortunately, there has been no widespread distribution of the tool, although the price per diagnosis is being realistically reduced. Even a continent-wide distribution, however, may still not be enough because the Xpert only diagnoses resistance to Rifampicin (which effectively all MDR-TB and XDR-TB is resistant to) - but not to any of the other drugs. At least a patient can immediately be recognised as being drug resistant, but the necessity for expensive culturing still remains for proper treatment if it is to be safe and effective.