Re: How are McFarland Units related to Nephelometry Turbidity Units?

Hi Chris:

I've had a poke around, and as near as I can tell there isn't a conversion factor between the two units. I think that's probably because turbidity is measured in rather "fuzzy" units. For instance, NTUs are a unitless ratio of absorbances (have a look at this question from our archives for more).

McFarland units are a turbidity-based measurement used to estimate of bacterial concentrations in cultures, and were developed by J. McFarland at the early part of the century. Like you said, he used Barium sulphate (the precipitate of adding barium chloride to dilute sulphuric acid) as a "standard" by which to count cells. It turns out that one "McFarland standard" (the precipitate of 100 μl of 1% barium chloride added to 9.9 ml 1% sulphuric acid) is roughly equivalent to 10⁸ cells/ml. So a "McFarland unit" isn't exactly a unit in the standard (i.e. SI) sense either.

If you look around at different nephelometers you'll find that most take measurements in different ways - different path lengths, different angles and different wavelengths, so the measurement of a sample can depend very much on the instrument being used. Another reason that a conversion factor won't work.

If it's really important that you have have McFarland units, I'd suggest making or purchasing some McFarland standards, and doing an empirical NTU:McFarland unit calibration for your instrument.

I hope that helps,
Rob Campbell, MAD Scientist

Reference:
McFarland J (1907) Nephelometer: an instrument for estimating the number of bacteria in suspensions used for calculating the opsonic index and for vaccines. Journal of the American Medical Association 14:1176-1178.