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.

 

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-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.

10 thoughts on “Dilutions: How Are You Doing Yours?”

  1. I was taught the “ratio” method for making dilutions. For example a 1:10 we were taught 1 part sample to 9 parts diluent to add up to the 10 which is the whole.

  2. Creo que el problema esta en la forma de estandarizar no solo en los Laboratorio inclusive se observado este en libros guías y protocolos de estudios . Es un punto de observación muy importante que lo realiza hay que tomarlos muy en cuenta al momento de realizar una dilución tanto a la muestra como preparación de los reactivos

  3. 30 Years in Lab, I first time read such concepts. I:2 or 1/2 we mean to say 1 part in total volume of 2, never listened or practised 1 ml add to 2 ml and say 1:2, we call it 1:3 or 1/3 and same is for 1:10 or 1/10. It is one part to 9 parts making the whole volume 10, I worked with many multi national pathologists in various labs, never such confusion arose.

  4. As a former molecular biology lab instructor, the terminology can seem confusing. A “1 to 10” could mostly likely be interpreted as adding 1 part of thing one to 10 parts of thing two totaling 11 parts but going back to the principle of a ratios, the use of 1:10 in dilutions is not the same as 1+10=11. Just like in math, 1:10 is treated as equal to 1/10 (as Mary mentioned). The “1 to 10” actually means the ratio of what you are adding to the total volume or “1 part of thing one to 10 parts total.” I always dedicated a section of my lesson to this concept to help aid students doing dilutions because it is not necessarily intuitive.

  5. Its should be stressed that writing 1:10 ought never to mean 1 part + 10 part = 11 parts total. That is simply incorrect, and indicates a lack of understanding of both the math and how to properly do a dilution.

  6. I think the confusion is more an Americanism than anything else. In Canada where I trained in MLS, 1:2 meant what it looks like as a ratio 1 part to 2 parts; for a total dilution of 1 in 3. I think the issue is using the proportional sign (: or “to”) interchangeably with “in.”

    To me “in” means a total volume of solution whereas “to” means a ratio of a certain number of parts of something to a certain number of parts of something else. Therefore I was taught that a 1/2 dilution is 1 part of A to 1 part of B making a total volume of 2. On the other hand, a 1:2 means 1 part of A added to 2 parts of B for a total dilution of 1 in 3 (a total volume of 3).

    I think it is simpler to use the convention most of the rest of the world uses where a fraction indicates “in” and a ratio sign (:) indicates “to.” In MLS a doubling dilution of serum would indicate a series of dilution with the final dilution expressed as the concentration of serum in that tube. So you would start with 1 part serum +1 part saline to produce a 1/2 solution, diluted 1/2 to give 1/4, then again to produce 1/8 etc. for example. The concentration (or titer) of the last dilution should be 1/8 or simply “8” but not 1:8.

  7. Dr. Patti Jones, I am sorry for my previous comments. I recalled my ratio lessons of Secondary School, and I realized my wrong concept. You r right, dilution should be written 1/2, 1/10 not 1:2 or 1:10. 1:2 denotes 1 to 2 parts and total 3. I checked our SOPs and manuals and luckily we have wrote 1/2, 1/3 and 1/10 every where. Thanks.

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