[ssorc]

A major reason why the accuracy on blood sugar strips is +/- 20% is because that is all that the standard (ISO 15197:2003) requires:

* Within ±0.83 mmol/L of lab results at concentrations of under 4.2 mmol/L

* Within ±20% of lab results at 4.2 mmol/L or more

Companies are juggling cost, reliability and performance/accuracy, with the first two generally winning in the marketplace as key selling points.

Some devices offer significantly better accuracy than 20%, depending on the market that you are in (not all are for sale in USA, IIRC).