working range
The working range is the band of concentrations a method is actually fit to measure — the comfortable stretch where it gives reliable, validated answers. Think of a recipe that turns out well only when you use between two and ten cups of flour: less and it won't bind, more and it overflows the pan. That trustworthy span is the working range.
Concretely, it runs from the lowest concentration the method can measure with acceptable precision and accuracy — near the limit of quantitation — up to the highest concentration it has been calibrated and validated for. Within this range the results meet the method's stated quality; outside it, performance is not guaranteed.
It matters because reporting a number from outside the working range is overstepping what the method can honestly support. If a sample reads above the top, you dilute it back into range; if it reads below the bottom, you may need to concentrate it or admit the result is only "less than" some value. Staying inside the working range is part of doing the measurement properly.
A glucose method is validated over a working range of 0.5 to 25 mmol/L. A blood sample reading 40 mmol/L is diluted twofold to about 20 mmol/L so the result falls inside the validated range.
Results are trustworthy only inside the validated working range.
The working range and the linear dynamic range often overlap but are not the same. The working range is defined by acceptable accuracy and precision and may include a curved upper portion fitted with a non-linear model; the linear range is strictly the straight-line part.