Technical Question....

robert ashcroft cytomat at
Wed Aug 5 07:23:36 EST 1998

Dear David,

Two factors are at work here,
the sampling error, determined by the size of your sample, and
the dead-time error, determined by the time it takes the machine to measure any 
one event, wherein it is "dead" to the measurement of any following event.

The sampling error goes as square root[N]/[N], and is 1% at 10,000 positives.

The dead-time error depends on how fast your machine can measure a cell and get 
ready for the next incoming cell.

It is easy to measure what that dead-time is, and then use Poisson statistics 
to compute the numbers of positives that are simply missed during measurement.

Typically, a dead-time of 0.1 ms will mean you miss about 30% of eligible cells 
in the sample at 10,000/s flow rates.

Let me know if you want to go further.

-----Original Message-----
From:	David Burke [SMTP:david at]
Sent:	Wednesday, August 05, 1998 3:56 AM
To:	Cytometry Mailing List
Subject:	Technical Question....

I recall some time ago that the number of positive events detected by flow 
cytometry does not directly correlate with actual number of cells. For 
instance, if 50,000 positive events are detected, can one accurately take that 
as 50,000 CELLS?

If not, what exactly is the deviation and how can the degree of deviation be 

Any references or helpful inputs????

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