# PharmPK Discussion - Reliable t1/2

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• On 18 Jul 2008 at 13:33:07, "Lee, Sun Ku" (SunKu.Lee.-a-.covance.com) sent the message
`Dear MembersThere is a little debate over confident terminal t1/2 values in non-compartmental analysis.  Let's say we have a PK data for 24 hr bloodcollection interval.  After non-compartmental analysis, we get theestimated t1/2 values as 30 hr.  In this case, is a 30 hr for t1/2reliable considering we only have 24 hr data?  If not, how can we seta reliable range of t1/2 values?  There are a couple of opinions onthat.  First, blood collection interval is the maximum reliable t1/2.For instance, for 24 hr blood collection interval, t1/2 values willnot be reliable over 24 hr.  Second, half of the blood collectioninterval is the maximum reliable t1/2.  The logic of this argument isthat we should be confident to see terminal phase after at least 50%of drug is eliminated.  In the same example, 12 hr will be maximumreliable t1/2.  Third opinion is to consider the time interval to beused in determining terminal phase.  Let's say we have a PK data for24 hr blood collection interval.  In non-compartmental analysis,terminal t1/2 was estimated using last three data point, i.e., 12 hr,16hr, 24 hr postdose.  The interval for terminal t1/2 selection is 12hr (24 hr - 12 hr).  In this case, reliable t1/2 will be 6 hr maximally.ThanksSun Ku Lee, Ph.D.Staff Scientist, PharmacokineticistPharmacokinetics and PharmacodynamicsCovance Laboratories`
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• On 21 Jul 2008 at 12:37:49, "Beumer, Jan H" (beumerjh.-a-.upmc.edu) sent the message
`The following message was posted to: PharmPKDear Sun,To accurately estimate a half-life, you should sample out to at least 4times the half-life into the terminal elimination phase. So a half-lifeof 24 h requires 96 h of sampling, corresponding to 93.75% excreted.Half-lives determined with sampling periods less than 4 times thehalf-life are not without value, but are less reliable, and you run therisk of missing a more slow phase of the elimination that only shows uplate in the curve. Later on in the development of a drug, when moresensitive assays are developed, and samples are taken for longer times,these slower phases are then discovered.Jan--Jan Hendrik BeumerPharm.D., Ph.D.Assistant Professor of Pharmaceutical SciencesSchool of PharmacyThe Hillman Cancer CenterResearch Pavilion, Suite G.27d5117 Centre AvenuePittsburgh, PA 15213-1863Email: beumerjh.at.upmc.eduhttp://www.pharmacy.pitt.edu/Directory/dir.lasso?Last=BeumerJ`
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