Introduction

A particular laboratory calculation that confuses us when we’ve not done it for a while is dilution. Accurate dilutions are essential to accurate medical laboratory test results. If you dilute something wrong, you will not get the correct result. Incorrect results could harm patients if the physician treats the patients based on those incorrect results.

I struggled with this infrequent calculation many times, as I only needed to think about new dilutions when implementing new test methods or training a new hire. Now that I am teaching Medical Laboratory Science, I explain dilutions frequently. The majority of students get a pained look on their faces and exclaim “Ugh! I hate trying to figure out dilutions!”

It’s really not that bad if you can remember that dilution factor and dilution ratio are different terms. I simplify it even further and call the former “dilution” and the latter “ratio”.

Some terms that will help as our discussion continues:

Reagent	a chemical or biological substance used to detect or measure the amount of a substance. Medical Laboratory Science uses this term specifically for these items used as part of our test kits or analyzers.
Specimen	blood, urine, or other body fluid from a human
Diluent	a liquid used to lower the concentration of a reagent or specimen to be used with a test kit or analyzer. An example would be saline or deionized water.
Analyte	the substance being measured by the test kit or analyzer
Normal saline	0.9% saline solution (sodium chloride solution), which is the concentration of saline in the human body.

Dilution vs. Ratio

A dilution is made by combining a certain volume of reagent or specimen with a certain volume of diluent.  The same can be said of a ratio. No wonder it’s confusing!

The difference is a dilution is expressed as parts reagent or specimen to total parts of solution. A ratio is expressed as parts reagent or specimen to parts diluent. Let’s look at some examples.

Examples of Standard dilutions

A standard dilution is a one-time event, mixing the reagent with diluent and using this solution in your analysis. 

We frequently dilute things times 2, which means 1 part reagent mixed with 1 part diluent. For example, I will mix 1 mL calcium reagent with 1 mL of deionized water to make a times 2 dilution. This is written as a 1:2 dilution or a times 2 (x2) dilution. The first number is the volume of reagent (1 mL) and the second number is the total volume of the final solution (2 mL). Expressing a x2 dilution as a ratio would be 1:1, or 1 mL reagent plus 1 mL water.

Let’s try another example. A medical laboratory scientist must dilute a specimen times 10 (x10) with normal saline prior to using it for analysis to ensure the analyte in the specimen is at the proper ratio to interact with the reagent optimally. (Oh, no! Dilution and ratio were used in the same sentence!) So, this 1:10 dilution is 1 part specimen to 10 total parts of solution, which if we need 10 mL of solution, equates to 1 mL specimen and 9 mL normal saline. The ratio is 1:9, or 1 mL specimen and 9 mL normal saline. 

As you can see in this example, the volume of specimen is the same in the dilution and ratio, and the volume of saline is the same in both expressions, as well. However, the way they are expressed is slightly different. You just need to understand the difference when explaining them. A 1:10 dilution is not the same as a 1:10 ratio. A 1:10 dilution is the same as a 1:9 ratio. 

Term	Dilution	Volume Reagent or Specimen	Volume Diluent
Dilution	1:9 1:10	1 part 1 part	8 parts 9 parts
Ratio	1:9 1:10	1 part 1 part	9 parts 10 parts

It is quite simple once you remember that a dilution is expressed in total parts of solution.

Why do we use the expression “dilution” in medical laboratory science more often than “ratio”, when it appears ratio is more logical when calculating how many parts of each substance to use? I like to think it’s because of the final calculations needed to determine the final amount of analyte in the specimen. If we use a 1:10 dilution, the result you get is multiplied by 10 to get the final result. For example:

A specimen is tested for cholesterol and diluted 1:10 prior to analysis.

The analyzer gives a result of 12 mg/dL on the diluted specimen, but does not do the calculation automatically to account for the dilution. You manually multiply it by 10, so your final cholesterol result is 120 mg/dL.

The dilution (x10) is the same factor you use to calculate the final result (x10).

If we used the equivalent ratio, it would be a 1:9 ratio. You must remember then to adjust your final calculation factor to the total number of parts in the dilution prior to multiplying. You would need to add 1 and 9 to arrive at 10 total parts, then multiply the result by the factor 10 to obtain the final result.

We need to know the total number of parts to use as the factor to determine the final result. It seems easier to determine that at the time of dilution rather than after analysis is complete. So, you can arrive at the same information in 2 different ways. The consensus among medical laboratory scientists is to account for it at the beginning of the process instead of the end.

Conclusion

Accurate dilutions are critical to accurate test results. This blog has hopefully explained the difference between dilution and ratio as it is used in medical laboratory science, improved the accuracy of your dilution calculations, and made it a less frustrating concept for you!