Two of the most consequential values on a test report are also two of the most misread: the limit of detection (LOD) and the limit of quantification (LOQ). They define the boundary of what a method can actually see, and they are the reason “not detected” does not mean “zero.” Here is what they really mean.
The floor of a measurement
No analytical method can measure arbitrarily small amounts. Below a certain level, a real signal becomes indistinguishable from noise. LOD and LOQ describe where those floors sit for a given method and matrix.
LOD: can we see it?
The limit of detection is the lowest amount of an analyte that a method can reliably distinguish from nothing, often defined around a signal-to-noise ratio of about 3:1. At the LOD you can say something is present, but not confidently say how much.
LOQ: can we measure it?
The limit of quantification is the lowest amount a method can measure with acceptable accuracy and precision, commonly around a 10:1 signal-to-noise ratio. Above the LOQ, a number is trustworthy; between LOD and LOQ, you can confirm presence but not reliably quantify.
Why “not detected” is not “zero”
“Not detected” means only that the analyte was below the method’s LOD. A sensitive method with a low LOD makes that a strong statement; a coarse method with a high LOD can report “not detected” while the substance is present just beneath its reach. That is why a “not detected” is only as meaningful as the LOD behind it.
Reading the result
Always read LOD and LOQ alongside any “none found” or low-level result. A contaminant result of “not detected (LOD 0.01 ppm)” is far more reassuring than “not detected” with no limit stated.
