When the sample volume becomes too small in a medical diagnostic.

In medical diagnostics, there is always the ambition to produce sensors with smaller and smaller volumes, especially in the example of capillary fill and microfluidic sensors.  One of the technical issues that can be run into is when the sample volume is too small relative to the biologically activity on the sensor and all the sample is consumed/or significantly depleted during the assay time.

We were recently developing an amperometry capillary fill enzymatic based sensor.  The assay functioned well when we pipetted a drop of test solution onto the sensor, but when we used a capillary fill chamber with a 250-micron spacer layer the assay did not perform so well.

We quickly made a spread sheet model and realised that the depletion layer, which was inevitable with this type of sensor, was diffusing across the 250-micron height of the chamber in about 20 seconds and causing assay problems.

We also created a calculator for calculating the height of a droplet on a surface, and estimated that for a 15-microliter drop pipetted onto a sensor we had approximately 650 micrometres of useful height for the depletion layer to diffuse up into. Therefore, we were able to explain the difference between whey pipetting drops onto the sensor gave a good signal quality but when we constrained the sample within a chamber with a height of 250 microns the signal quality diminished.

The take home message for us was: the assay was too active for such a thin layer of solution.  In general, smaller sample volumes are more prone to compositional changes caused by-products from biochemical reactions and species generated by the counter electrode. As with everything there is a trade-off between smaller volumes being beneficial to the patient and providing compelling product claims, and the effect that smaller volumes can have on the signal quality.

If this sounds like something you could also be experiencing, then feel free to contact us.