Method validation of circulating tumour cell enumeration at low cell counts

Circulating tumour cells (CTC) are receiving increasing attention as prognostic, predictive and pharmacodynamic biomarkers in cancer patients. However, their clinical significance can be dependent on an accurate determination of CTC around cut-off values at low cell counts (<10 cells/7.5 ml). Consequently, we have conducted method validation of the CellSearch™ system focusing on clinical samples containing CTC in the cut-off region. Analytical accuracy was first assessed employing quality controls (QC) and spiked healthy volunteer blood specimens. Results were analysed by β-expectation tolerance intervals (BETI). Inter-operator error (6 different readers) was then characterised in 38 different patient samples, 68% of which had ≤5 CTC and data were analysed by β-content γ-confidence tolerance intervals (BCTI). Results from QCs and spiked blood confirmed a 3-4-fold higher degree of imprecision at the low (48 cells, BETI = + 0.288/-0.345, β = 95%) compared to the high QC (987 cells, BETI = +0.065/-0.140, β = 95%). However, when data for individual analysts were interrogated characteristic systematic errors were detected. In the analysis of patient samples again individual analysts introduced a highly specific error into the interpretation of CTC images, which correlated to the level of training and experience. When readers were selected based on BETI and BCTI results, the high level of between-operator error (up to 170%) observed at CTC of ≤ 5 was reduced to < 30%. Inter-operator variability in enumeration of CTC at low cell counts can be considerable, but is also potentially avoidable by following simple guidance steps