Just thought I'd drop my thoughts into this one: So far I've seen a lot of discussion about the positives, but none about controls. I've always been taught that a Flow Cytometer doesn't measure absolutes, but the ratio between control and positive samples. This being the case, if a positive population can be put into the third or fourth decade as suggested then the control would be correspondingly raised and the ratio remain constant. This, I believe, is the factor which will give repeatability, not keeping the instrument settings the same. BTW I have seen intensity times number used to give a meaningful answer, but in histology, where immunostained slides can be assessed to give a value for oestrogen receptor levels which is comparable to an ELISA assay. Eric P Miller Edinburgh Medical Oncology Unit "Worrying is very effective. 90% of the things I worry about never happen."
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