# Understanding Widal Test Kit Dilutions

• TytoAlba95
In summary, the conversation is discussing the concentrations and dilution factors used in the Himedia Widal test teaching kit. The numbers written on top of the test tubes represent the dilution factor, which is equal to the final volume divided by the aliquot volume. The conversation also mentions a typographical error in the teaching kit, which has since been corrected on the company's website.
TytoAlba95

This snip off is from Himedia Widal test teaching kit.

I understand 1/20 1/40 1/80... are the concentrations in each test tube. They are obtained by the following calculation: (2nd testube) 1/2 X 1/20.
But I can't decipher the numbers written at the top, 10-1 , 10-2 etc.

Besides which of of these two set of numbers should be called dilution factor.

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That should be ##2^{-n}## for different n. Maybe it was copied from a different problem where the dilution was a factor 10 each step.

TytoAlba95
Welcome to PF.
I think they have used old numbers from steps of 0.1 when they should now use steps of 0.5
Following the dilution, the concentration in the tubes will be;
#1, 1:20 = 0.05 = 0.1 × 2-1
#2, 1:40 = 0.025 = 0.1 × 2-2
#3, 1:80 = 0.0125 = 0.1 × 2-3
#4, 1:160 = 0.00625 = 0.1 × 2-4
#5, 1:320 = 0.003125 = 0.1 × 2-5
#6, 1:640 = 0.0015625 = 0.1 × 2-6
#7, 1:1280 = 0.00078125 = 0.1 × 2-7
In each case the 0.1 = 10-1 = initial dilution of 10.

TytoAlba95
Which number here could be called dilution factor?

Thanks

The dilution factor is equal to the final volume divided by the aliquot volume.
For tube #1; 1:20; The dilution factor is 2.0 mL / 0.1 mL = 20.
The dilution factors for tubes #1 to #7 are 20, 40, 80, 160, 320, 640 and 1280.

TytoAlba95
Baluncore said:
The dilution factor is equal to the final volume divided by the aliquot volume.
Thanks

... Kindly note that, there was a typographical error in our TD (diagram for dilutions in Page No 5 of HTI018) which has been uploaded in our website. ...
The new sheet on the website has removed the offending numbers.

mfb and TytoAlba95
Merlin3189 said:

The new sheet on the website has removed the offending numbers.
Oh thank you.

## 1. What is a Widal test kit and why is it used?

A Widal test kit is a diagnostic tool used to detect the presence of certain antibodies in the blood that indicate past or current infection with the bacteria that cause typhoid fever. It is commonly used in areas where typhoid fever is prevalent, as well as in outbreak investigations and for surveillance purposes.

## 2. How does the Widal test work?

The Widal test works by mixing a patient's blood sample with a series of dilutions of the bacteria that cause typhoid fever. If the patient has been exposed to the bacteria, their blood will contain antibodies that will react with the bacteria in the test tubes. The reaction is then observed and interpreted to determine the level of antibodies present in the blood.

## 3. What dilutions are typically used in a Widal test?

The most commonly used dilutions in a Widal test are 1:20, 1:40, 1:80, and 1:160. These dilutions are used to determine the presence and levels of antibodies against two specific antigens of the typhoid bacteria, O and H. The highest dilution at which the antibodies can still be detected is considered the titer, and this is used to interpret the results of the test.

## 4. How accurate is the Widal test?

The accuracy of the Widal test can vary depending on a number of factors, including the experience and technique of the person performing the test, the quality of the test kit, and the prevalence of typhoid fever in the population being tested. In general, the Widal test is considered to have a moderate level of accuracy, and it is often used in combination with other diagnostic tests for typhoid fever.

## 5. Are there any limitations or drawbacks to the Widal test?

Yes, there are some limitations and drawbacks to the Widal test. One major limitation is that it can produce false positive results in people who have been previously vaccinated against typhoid fever or have had a previous infection with a similar bacteria. Additionally, the test may not be able to distinguish between a current infection and a past infection, as antibodies can persist in the blood for a long time after the infection has resolved. It is important for the Widal test to be interpreted in conjunction with other clinical and laboratory findings to accurately diagnose typhoid fever.

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