Inter-laboratory comparison of DNA flow cytometric results from paraffin-embedded breast carcinomas.

Consecutive sections from 33 paraffin-embedded human breast carcinomas without intratumor heterogeneity were sent for flow cytometric (FCM) DNA analysis in two experienced laboratories. FCM instruments, run conditions, and tumor disaggregation procedures were different in the two laboratories. In four cases (12%) the laboratories reported a different DNA ploidy and DNA index (DI). These variations were due to analytical reasons, differences in the detection rates of near-diploid and tetraploid DIs, not due to interpretation of data or the criteria used for aneuploidy. There was a significant correlation between S-phase fractions (SPF) obtained in the two laboratories (r = 0.90, p less than 0.0001) if only cases with concordant DI were included. Discordant DI usually led to very different SPF values.