1. MLAB 1335 Immunology/Serology Terry Kotrla
Serial Dilutions
MLAB 1335 Immunology/Serology
Terry Kotrla

2. Introduction
Many of the laboratory procedures involve the use of dilutions.
It is important to understand the concept of dilutions, since they are a handy tool used throughout all areas of the clinical laboratory.
These dilutions have to be considered as they make a quantitative difference in what is going on.

3. Serial Dilutions
A serial dilution is any dilution where the concentration decreases by the same quantity in each successive step.
Serial dilutions are mutiplicative.

4. What Does This Mean??
If a solution has a 1/10 dilution the number represents 1 part of the patient sample added to 9 parts of diluent.
So the volumes used would be 10-1= 9.
This represents 1 part patient sample added to 9 parts of diluent.

5. Dilutions
If a 1/8 dilution of the stock solution is made followed by a 1/6 dilution what is the final dilution.
The final dilution is: 1/8 x 1/6 = 1/48
These type of dilutions are trickier and not used very frequently in the clinical lab.

6. Doubling Dilutions “Doubling dilutions” are very popular.
This is a series of ½ dilutions. Each successive tube will ½ the amount of the original concentrated solution.
If this is done 6 times this is what you would end up with:

7. Doubling Dilution 6 Times
1st dilution = 1 /2
2nd dilution = 1 /2 x 1 /2 = 1/4
3rd dilution = 1/4 x 1 /2 = 1/8
4th dilution = 1/8 x 1 /2 = 1/16
5th dilution = 1/16 x 1 /2 - 1/32
6th dilution = 1/32 x 1 /2 = 1/64
This results in a series of dilutions, each a doubling dilution of the previous one

8. Dilution Factor
The dilution factor is the final uses the formula volume/aliquot volume.
EXAMPLE: What is the dilution factor if you add 0.1 mL aliquot of a specimen to 9.9 mL of diluent?
The final volume is equal to the aliquot volume PLUS the diluent volume: mL mL = 10 mL
The dilution factor is equal to the final volume divided by the aliquot volume: 10 mL/0.1 mL = 1:100 dilution

9. Practice
Problem: What is the dilution factor when 0.2 mL is added to 3.8 mL diluent?

10. Set Up The Problem dilution factor = final volume/aliquot volume
= 4.0 total volume
4.0/0.2 = 1:20 dilution

11. Problem Continued
Remember that serial dilutions are always made by taking a set quantity of the initial dilution and adding it successively to tubes with the same volume.
So each successive dilution would be multiplied by the dilution factor.

12. Problem Continued
So in the above problem all successive tubes would have 3.8 mLs of diluent.
You would then transfer 0.2 of the initial diluted sample into the next tube, mix transfer 0.2, mix and so on.
If you had 4 tubes what would be the final dilution of tube 4?

13. Solving the Problem - *Calculate DF of tube 12 3 4
Aliquot
0.2
Diluent
3.8
Math
*4/0.2
1/20x1/20
1/400x1/20
1/8000x1/20
Dilution
1:20
1:400
1:8000
1:160,000

14. Solving the Problem
Or if you simply wanted to know the dilution of the final tube you could just multiply them together:
1/20 x 1/20 x 1/20 x 1/20 = 1:160,000

15. Next What if we added the indicator system to the test system?
You must create additional rows, if you are showing your work, to take into consideration the dilutional effect that will have on the dilution.
When an indicator is added it is CRITICAL to remove an aliquot from the last tube.

16. Adding Indicator Cells
Tube 1 2 3 4
Dilution
1:20
1:400
1:8000
1:160,000
RBCs
0.2
Volume
3.8 DF
4/1
Math
4x20
4x400
4x8000
4x160,000
1:80
1:800
1:32,000
1:640,000

17. Serial Dilutions
When performing serial dilutions in the clinical laboratory one must know the dilution of each tube so a chart is always created to indicate the dilution of each tube.
If you know the dilution factor you multiply each successive tube by it and can easily determine the concentration of each tube to create your chart.

18. Titers
TITERS are reported out as the reciprocal of the last tube giving a positive reaction.
So if tube 2 was the endpoint, the dilution is 1:800 the titer is reported out as 800/1= 800.

19. Other Applications
Sometimes when analyzing a blood sample the results are above the linearity (or accuracy) of the machine.
When this happens the blood must be diluted, retested, then the result multiplied by the dilution factor.

20. Example A blood glucose of 800 mg/dL was obtained.
According to the manufacturer the highest glucose result which can be obtained on this particular instrument is 500 mg/dL.
The sample must be diluted.

21. Example The serum was diluted 1:10 and retested.
The result is 80 mg/dL.
THIS IS NOT THE REPORTALBE RESULT!
You must multiply by the dilution factor of 10.
10 x 80 = 800 mg/dL