Document ID: ./input/supremecourt_opinions/opinions/12pdf/11-1425_cb8e.pdf
Page Number: 13.0

Cite as:  569 U. S. ____ (2013) 

9 

Opinion of the Court 

771.  Testimony before the trial court in this case indicated 
that  the  percentage  of  alcohol  in  an  individual’s  blood
typically decreases by approximately 0.015 percent to 0.02
percent per hour once the alcohol has been fully absorbed.
App.  47.  More  precise  calculations  of  the  rate  at  which
alcohol dissipates depend on various individual character­
istics  (such  as  weight,  gender,  and  alcohol  tolerance)  and 
the  circumstances  in  which  the  alcohol  was  consumed. 
See Stripp, Forensic and Clinical Issues in Alcohol Analy­
sis, in Forensic Chemistry Handbook 437–441 (L. Kobilin­
sky ed. 2012).  Regardless of the exact elimination rate, it 
is sufficient for our purposes to note that because an indi­
vidual’s alcohol level gradually declines soon after he stops
drinking,  a  significant  delay  in  testing  will  negatively 
affect  the  probative  value  of  the  results.    This  fact  was 
essential  to  our  holding  in  Schmerber,  as  we  recognized
that,  under  the  circumstances,  further  delay  in  order  to
secure  a  warrant  after  the  time  spent  investigating  the 
scene of the accident and transporting the injured suspect
to the hospital to receive treatment would have threatened 
the destruction of evidence.  384 U. S., at 770–771. 

But it does not follow that we should depart from careful
case-by-case  assessment  of  exigency  and  adopt  the  cate­
gorical rule proposed by the State and its amici.  In those 
drunk-driving  investigations  where  police  officers  can 
reasonably obtain a warrant before a blood sample can be
drawn  without  significantly  undermining  the  efficacy  of
the  search,  the  Fourth  Amendment  mandates  that  they
do so.  See McDonald v. United States, 335 U. S. 451, 456 
(1948)  (“We  cannot  . . .  excuse  the  absence  of  a  search 
warrant  without  a  showing  by  those  who  seek  exemption 
from the constitutional mandate that the exigencies of the
situation made [the search] imperative”).  We do not doubt 
that  some  circumstances  will  make  obtaining  a  warrant
impractical  such  that  the  dissipation  of  alcohol  from  the 
bloodstream will support an exigency justifying a properly