Method validation of circulating tumour cell enumeration at low cell counts

BACKGROUND: 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. METHODS: 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: 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%. CONCLUSIONS: Inter-operator variability in enumeration of CTC at low cell counts can be considerable, but is also potentially avoidable by following simple guidance steps

Optimisation of an immunohistochemistry method for the determination of androgen receptor expression levels in circulating tumour cells

BACKGROUND: AZD3514 inhibits and down regulates the androgen receptor (AR) and has undergone clinical trials in prostate cancer. To provide proof-of-mechanism (POM) in patients, an immunohistochemistry (IHC) method for determination of AR in circulating tumour cells (CTC) was developed and validated. METHODS: After an assessment of specificity validation focused on intra- and inter-operator reproducibility utilising a novel modification of incurred sample reanalysis (ISR). β-Content γ-confidence tolerance intervals (BCTI) and Cohen’s Kappa (κ) were employed in statistical analysis of results. RESULTS: In a first set of IHC reproducibility experiments, almost perfect agreement was recorded (κ=0.94) when two different operators scored CTC as overall positive or negative for AR. However, BCTI analysis identified a specific bias in scoring staining intensity, where one operator favoured moderate over strong assignments, whereas the reverse was the case with the second operator. After a period of additional training involving deployment of a panel of standardised images, a second set of validation experiments were conducted. These showed correction of the inter-operator bias by BCTI with κ for scoring intensity increasing from 0.59 to 0.81, indicative of almost perfect agreement. CONCLUSIONS: By application of BCTI to the validation of IHC, operator bias and therefore poor reproducibility can be identified, characterised and corrected to achieve a level of error normally associated with a quantitative biomarker assay, such as an ELISA. The methodological approach described herein can be applied to any generic IHC technique