LoB = limit of blank
LoD = limit of detection
LoQ = limit of quantification
Although there are various ways to calculate these quantities, LoB can be estimated by running some samples without analyte and adding 2 sd to the average. LoD is estimated by running some low samples and adding 2 sd to the LoB. LoQ is estimated by running some low samples and calling LoQ where the data equals a pre specified CV. See here for some pictures.
I’m not sure how most laboratories handle reporting values below the detection limit (LoD) but CLSI EP17-A2 suggests that for values between the LoB and LoQ, the value should be reported as “analyte detected, result < LoQ.”
Some of my blog entries (such as this one) come from my consulting experience, which is of course confidential. So here is a made up example of a tumor marker with a detection limit of 1.0 and some patients who are being serially monitored. As one might expect, no tumor marker is an ideal result.
Patient A: 0.3, 0.6, 0.2, 0.6, 0.3
Patient B: 0.2, 0.5, 0.4 0.6, 0.8
These quantitative results could be compared to the reports for all values of “analyte detected, result < LoQ.”
Clearly, there is something happening with patient B and this trend information is lost by the reporting rules.