Characterisation of inter-operator variability in CTC enumeration at low cell counts utilising BCTI and patient samples. Image galleries generated from the analysis of different cancer patient blood samples, 68% of which had ≤5 CTC, were enumerated by a pool of operators. Results were analysed by a modification of incurred sample reproducibility where the counts obtained by a pair of operators who interrogated the same samples were substituted into the calculations. Tolerance intervals (±) were constructed at β = 95% and 67% and the ± 30% error line is shown for reference. (a), Analyst 1 versus analysts 2–6; (b), analyst 2 versus analysts 3–6; (c) analyst 3 versus 4–6 and (d) analyst 4 versus 5 and 6 and analyst 5 versus 6.