STATISTICS
Blank results Low-concentration results LoB 0.16 Illustrative LoD 0.32
A claimed limit must
describe the complete measurement
0.0 0.1 0.2 0.3 0.4 Reported concentration (arbitrary units)
Fig 1. Blank and low concentration result distributions in the simulated example. The LoB is a threshold derived from blank results. The LoD is the concentration distribution expected to exceed that threshold with the chosen probability.
Materials and replication
For LoB, use more than one suitable analyte-free material where the matrix can influence background signal. Replication should be distributed rather than produced in one uninterrupted run. For LoD, prepare/reuse stored low level samples around the expected level. Relying on one concentration can conceal a poor estimate or a changing variance. For LoQ, test several levels below and above the anticipated limit so that the performance profile crosses the acceptance boundary. Independent preparations are preferable to repeated aspiration of one vial when preparation contributes materially to variation. Patient samples are often the ideal matrix but may be difficult to obtain at exact low concentrations. Pools, dilutions or spiked materials can be justified, provided their limitations are recognised, risk assessed and recorded. Dilution can alter matrix composition. Spiking can produce an analyte form unlike that in patient samples. The next article in this series (Part 6) considers matrix effects and commutability in more detail.
Data inspection and analysis Plot the individual results by material, concentration, run and day before calculating a limit. A histogram or distribution of blank results reveals skewness and censoring. A scaterplot of standard deviation against concentration shows whether constant variance is plausible. A detection rate plot shows the proportion exceeding the LoB at each low concentration. A precision or error profile shows where the chosen quantitative criteria are met.
October 2026
WWW.PATHOLOGYINPRACTICE.COM 19 0.5 0.6
procedure under stated conditions, not merely the instrument’s ability to register a signal
Relative frequency
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