Someone in the lab emailed me today saying that she wouldn't be coming in as she was ill, and that she hoped it was a cold instead of flu. If you're in the same situation, wouldn't it be good if you could test yourself to find out? Well, you can, using a simple dip-stick style test which you can buy online.
That's great and it's easy to envisage how useful this can be but, for me, the interesting page is the one describing the test specifications - how good is the test? The most notable values are the positive (PPV) and negative (NPV) predictive values, which represent the accuracy of a result; PPV represents the proportion of positive results that are truly positive and likewise the NPV is the proportion of test negatives that are truly negative. A PPV of 62% for a nasal swab doesn't seem to be particularly high. As these values very much rely upon seroprevalence it's hard to really tell how useful this is.
These assays do work, and simple lateral flow devices worked really well during the foot and mouth outbreak in 2007 in the UK. These assays currently use antibodies and, without a good antibody, assay sensitivity can be poor. In lab settings, molecular assays based upon detecting viral nucleic acid are generally more sensitive, in particular real-time RT-PCR. Sadly, molecular methods also require much more expensive equipment, making it more difficult to convert a lab procedure into a 'point-of-care' test. Perhaps unsurprisingly, the initial major steps were taken by the military as a response to the threat of biowarfare. Now however, more commercial machines have made their way to the market, for example machines made by Smiths detection.
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| Smiths detection PCR machine: essentially a robot for nucleic acid extraction and a real-time PCR machine packaged in a (very heavy and expensive) briefcase. |
Field based diagnosis is as close now as it has ever been. Ultimately though, the problems of hardware still exist. The machines are expensive; essentially they are the same machines that a lab has packaged into a briefcase. I think molecular assays in the field will only really take off once isothermal assays, such as loop-mediated isothermal amplification (LAMP), are more widespread. As their name suggests, isothermal assays are performed at a single temperature, so all that is required is a simple waterbath set to a particular temperature, as opposed to a block of metal heating up and cooling down.
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| Loop mediated isothermal amplification: modified primers loop back to prime the alternative strand. A strand displacing enzyme abrogates the need for variable temperatures. |
Detection is the other problem; fluorescence as a read-out is expensive to a) achieve and b) detect, therefore the gold standard for field-based detection of amplification (PCR, LAMP etc.) is likely to be dipsticks/lateral flow devices as these are easy, more foolproof and provide a clear answer. For now, extraction of the nucleic acid probably remains the biggest obstacle. Another benefit of assays based around isothermal amplification and simple methods of detection is that it allows the use of molecular assays with minimal equipment, meaning that such assays can then be used in resource poor countries.
It is more than likely that the future will be much more sophisticated with much greater scope for what can be detected, although the requirements for simplicity are unlikely to change much. For now, imagine settings such as clinics where patients suspected of hepatitis C can get an instant result. LAMP assays are already available for many of the pathogens that would be on a list of desirable tests; I suspect it won't be long before point of care testing becomes even more widespread than it already is.
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