Measurement of viruses by end-point dilution assay

13 July 2009

The plaque assay is a terrific method for determining virus titers, but it doesn’t work for all viruses. Fortunately there are several alternative methods available, including the end-point dilution assay.

The end-point dilution assay was used to measure virus titer before the development of the plaque assay, and is still used for viruses that do not form plaques. Serial dilutions of a virus stock are prepared and inoculated onto replicate cell cultures, often in multi-well formats (e.g. 96 well plastic plates). The number of cell cultures that are infected is then determined for each virus dilution, usually by looking for cytopathic effect.

In this example of an end-point dilution assay, 10 monolayer cell cultures were infected with each virus dilution. After an incubation period, plates that displayed cytopathic effects were scored with a +. At high dilutions, none of the cell cultures are infected because no particles are present. At low dilutions, every cell culture is infected. Half of the cell cultures showed cytopathic effects at the 10-5 dilution. This is the end point: the dilution of virus at which 50% of the cell cultures are infected. This number can be calculated from the data and expressed as 50% infectious dose (ID50) per milliliter. The virus stock in this example contains 105 ID50 per ml.


In real life, the 50% end point does not usually fall exactly on a dilution as shown in the example. Therefore statistical procedures are used to calculate the end point of the titration.

End-point dilution methods can also be used to determine the virulence of a virus in animals. The same approach is used: serial dilutions of viruses are made and inoculated into multiple test animals. Infection of the animal can be determined by death or clinical symptoms such as fever, weight loss, or paralysis. The results are expressed as 50% lethal dose (LD50) per ml or 50% paralytic dose (PD50) per ml when lethality or paralysis are used as end points.

The following example illustrates the use of end point dilution to measure the lethality of poliovirus in mice. Eight mice were inoculated per virus dilution, and the end point was death. The statistical method of Reed and Muench was used to determine the 50% end point. In this method, the results are pooled, and the mortality at each dilution is calculated. The 50% end point, which falls between the fifth and sixth dilutions, is calculated to be 10-6.5. Therefore the virus sample contains 106.5 LD50 units.


Reed, L.J., & Muench, H. (1938). A simple method of estimating fifty percent endpoints. Am. J. Hygiene, 27, 493-497

  • phuongthuy

    Dear sir.
    Im VietNamese. I am studying about Varicella virus. I want to determine the titer of virus in vaccine but
    I face a problem when choose the media diluent virus and the media for overlay. Can you help me?  My Thank you.

  • profvrr

    My colleague Saul Silverstein, who worked on varicella virus sent me the following: To titrate virus dilute in PBS containing 2% calf serum and overlay a cell monolayer with a minimal volume of liquid (100ul for a 3.5 cm dish), incubate at 37 and gently tap the sides of the dish even 15 min for 1 hr. After 1hr remove virus and add 3mls of medium with serum and incubate. You should see plaques developing in a few days. You can add 0.1% pooled human gamma globulin to reduce plaque spread. Remove medium, fix with methanol and stain with crystal violet.

  • aditya asopa

    great job. Would you please tell us about Karber’s method.?

  • Phuongthuy

    Thank you very much. But I think the medium with serum that add after remove virus and incubate is very imfortant. Can you tell me which is medium? MEM or other? Thank you.

  • Thuyniheje

    I am in Viet Nam. I know the Reed and Muench. If you want to know it, I can give to you. Contact me:

  • Kamola

    I am making TCID 50, what does it mean, make serial dilutions from virus stock, is it virus supernatant from  third amplification step, or purified virus recovered from supernatant by ultracentrifugation. Purified virus is stored in storage medium containing glycerol, can this somehow affect the assay. the protocol i’m using requires me to take 200ul of virus (not sure, the supernatant or the purified virus), if its purified virus, its too much, my cells will die in 2 days, can i some how minimize, the volume?

  • Shahzaduaar772

    i am here in pakistan a phd student. i need original paper of reed and Reed L.J. & Muench M. (1938) A simple method of estimatingfifty percent end points. American Journal of Hygiene
    Reed L.J. & Muench M. (1938) A simple method of estimating
    fifty percent end points. American Journal of Hygiene
    27, 493–497. for calculation of LD 50.493–497. for calculation of LD 50.


  • Sgalattas

    What is the equation 4 TCID50 ? How can i explain it to my students ?

  • Rama

    Hi friends!
    Read the  following. Really helpful in understanding TCID50. Further, I send my new formula for the publication and will upload after acceptance.

    M. A. Ramakrishnan, Senior Scientist, Indian Veterinary Research Institute, Mukteswar, India 
    maramakrishnan@gmail:disqus .com

  • Nisha D

    Hi Dr. Racaniello,

    I was reading a paper where they inactivated A/PC using gamma radiation. When they immunized mice in their study they claim that they used doses of 3.2 x 10^6 PFU equivalents. Is the term “PFU equivalent” widely used and what does it actually mean? Would you be able to explain the process of quantifying inactivated virus? Is there a way to use something like a TCID50 equivalent?

    Thanks for your help and time!


  • Anthony

    “The 50% end point, which falls between the fifth and sixth dilutions, is calculated to be 10-6.5.” (from the last paragraph)

    isn’t it 6th and 7th dilution?

  • Winasavitri

    Hello Prof!
    I’m a master student working on viroid. Could I use this method for viroid?

  • Sonalika Mahajan

    Hello Sir,
    I am doing PhD in Indian Veterinary Research Institute on Foot-and-Mouth disease virus. Can u please send me the original paper by Reed and Muench on TCID50. I’ll be highly obliged. my email id is sonalikam1@gmail.comReed, L.J., & Muench, H. (1938). A simple method of estimating fifty percent endpoints.Am. J. Hygiene, 27, 493-497 

  • Lolw84

     I would like to know weather TCID50 can be used to evaluate antiviral activity of  plants against Measle , Mumps  ,Rubella viruses.

  • Rohan

    what do you mean by 100 TCID5 0 ?

  • Rohan

    Dear Dr Vincent,
                                       Please send me  the original Reed and Muench paper on TCID50. Im a virology student and i think this paper would be the ideal way for me to understand the basics. Thankyou

  • Rohan
  • ZA

    can anyone send me the paper please?
    zcbec03                                thank you! :)

  • HZ

    Can anyone send me the original paper of Reed L.J. & Muench M. (1938) A simple method of estimating fifty percent end points. I am looking for the paper by Reed and Muench on the internet and our school library but couldn’t find it. My email is Appreciate for your help

  • knh1221

    Dear Sir.

    I have a question about MOI. Some paper says that ‘PK15 cells were infected with PCV2(virus) at an MOI of 1 TCID50 per cell’. I don’t know what’s mean ‘per cell’. Could you please explain about that. Thank you

  • Zawar

    Sir am a Bachelors student of 3rd Semester from Pakistan…I want to know that Does the use of Different virus inocculum in different dilutions effects the final Titer of virus harvest….? Need help on that…And if it does than how and why? here is my email id

    Will be thankful :)

  • Ramakrishnan Muthannan Andavar