# PharmPK Discussion - T1/2 and MRT relationship

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• On 19 Mar 2010 at 12:57:13, phar_res (phar_res1.-at-.yahoo.com) sent the message
`Dear All,Considering t1/2 = 0.693 X Vdss/CL = 0.693 X MRT,MRT should always be 1.5 times higher than t1/2. But in many PK publications, I have seen this relationship going off by a large margin. Is it just because of an inappropriate experimental design? ThanksSunil`
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• On 23 Mar 2010 at 11:05:18, Norman Z (freecg.aaa.gmail.com) sent the message
`Hi Sunli, I think the equation t1/2 = 0.693 X Vdss/CL = 0.693 X MRT is only true if the drug follows one compartment model. For more complicated model, the t1/2 people report may be the terminal half life, which is different from t1/2 for a one compartment model. If they also use non-compartment analysis to deal with the data, the MRT will not be 1.5 times of t1/2. Thanks, Norman --Norman Zhou NSEC fellow, The Ohio State University College of Pharmacy`
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• On 24 Mar 2010 at 11:23:39, "Davies, Brian" (brian.davies.-at-.roche.com) sent the message
`The following message was posted to: PharmPKDear NormanI think that you have got things a little bit complicated. The equation 0.693 x MRT can apply to drugs showing one compartment or multiple compartments and is probably the best half-life to use when assessing rise to steady-state for a multiple compartment drug. For a drug showing one compartment characteristics then 0.693 x MRT will be the terminal half-life.The use of 0.693 x MRT is particularly useful for drugs showing diffusion rate limited kinetics where the rate constant associated with the first phase is the elimination half-life and the terminal half life reflects the transfer of drug from the peripheral compartment and is usually close in value to the micro rate constant k21. The half-life calculated from 0.693 x MRT is the effective half life - I probably shouldn't have opened that subject again!!!regardsBrian`
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