Dilutions: How Are You Doing Yours?

If you ask someone to dilute a sample in half, pretty much everyone will do it the same way – add an equal volume of sample to an equal volume of diluent, whether that’s 1 mL to 1 mL or 100 µL to 100 µL. But if you ask people to do a 1 to 2 dilution, you may be surprised to get different results. That’s because I’ve found that the convention for writing dilutions is taught differently at different Medical Laboratory Science (MLS) schools.

A 1 to 2 dilution should be written as ½. It means to dilute something in half. But many times it will be written as 1:2. These two forms are actually not equal, despite the fact that they are used interchangeably in the laboratory. One is a dilution and the other is a ratio. In the scientific literature, if you see “1:2”, it means to add 1part to 2 parts. That will be 1 mL added to 2 mL, for a total of 3 mL, or a 1/3 dilution.

Unfortunately, this problem is prevalent in the laboratory. I’ve seen 1 to 10 dilutions written both as 1/10 and 1:10.   It’s very important to know how the technologists in the lab are performing that 1 to 10 dilution. Are they doing a true 1/10 (1 mL sample plus 9 mL diluent) or are they actually doing a 1 to 11 dilution (1 mL sample plus 10 mL diluent)? Your patient results may be different depending on who does the dilution!

Coming into this field from a scientific background rather than an MLS background, I prefer the convention of writing a dilution as 1 over something, ½, 1/10, rather than as a ratio, 1:2, 1:10. However, perhaps the majority of medical laboratory scientists are taught the ratio. Either convention works fine as long as it is clear to everyone in the lab what dilution they are actually performing and being asked to perform.  You might want to just check your own MLS and see how they do their dilutions.



-Patti Jones PhD, DABCC, FACB, is the Clinical Director of the Chemistry and Metabolic Disease Laboratories at Children’s Medical Center in Dallas, TX and a Professor of Pathology at University of Texas Southwestern Medical Center in Dallas.