Serial dilutions involve making an initial dilution of an original sample in an unused container and then making a secondary dilution of the initial diluted sample into a new unused container. This process is repeated on each new diluted sample until the overall total dilution is reached. For titers, a portion of diluted material must remain from each dilution set in order to test each dilution to identify the end of detectable analyte.
Serial dilutions are useful in many areas of the clinical laboratory particularly because they limit the amount of needed sample and diluent, even when performing high levels of dilution. An example would be if we wanted to create a 1 to 10,000 dilution on 0.10 mL of an original serum sample. To complete this in a single dilution, 999.9 mL of diluent would be needed (figure 1). In contrast, creating 4 serial dilutions of 1 to 10 would only require 3.6 mL of total diluent (figure 2).
In serial dilutions, the total dilution can be calculated by multiplying each individual dilution together.
Here are some examples of how the total dilution can be calculated:
- 4 serial dilutions which are each at a "1 to 2" dilution would create a total dilution of 1 to 16 in the last container.
2 x 2 x 2 x 2 = 16
- 3 serial dilutions which are each at a "1 to 10" dilution would create a total dilution of 1 to 1,000 in the last container.
10 x 10 x 10 = 1,000
- 5 serial dilutions which are each at a "1 to 5" dilution would create a total dilution of 1 to 3,125 in the last container.
5 x 5 x 5 x 5 x 5 = 3,125
