Answering your questions

April 1, 2009
Max detection time for ETOH

Q What is the maximum detection time for ETOH?
Any new tests available for legal testing?

A
The average adult eliminates ethanol at a rate of
15 mg/dL/h to 20 mg/dL/h (7 g/h to10 g/h, or the equivalent of about one
drink per hour) with a wide range between individuals of 10 mg/dL/h to
34 mg/dL/h.1 Much of this pharmacokinetic variability is a
result of genetic and environmental factors including age, gender, and
history of chronic ethanol use. Specifically, women metabolize ethanol
slightly faster than men (18 mg/dL/h compared to 16 mg/dL/h),2
while chronic alcohol abusers have very high elimination rates (30 mg/dL/h
to 40 mg/dL/h) due to autoinduction of the metabolizing enzyme CYP2E1.
In addition to pharmacokinetic variability, maximum detection times are
also affected by the choice of specimen and analyte. Maximum detection
times among blood, urine, and saliva samples range between eight hours
and 24 hours for ethanol. In contrast, the detection times for ethanol
metabolite are much longer. Ethyl glucuronide can be detected in urine
up to one to five days after consumption, and ethyl glucuronide and
fatty-acid ethyl ester metabolites can be detected in hair samples for
up to 90 days after ingestion.

Several recent trends and developments in ethanol
testing will likely impact future legal testing. First, more companies are
selling point-of-care breathalyzers and saliva tests for home/private
testing. While some of these test kits are Department of Transportation
approved, most have not been approved by the Food and Drug Administration
nor do they have established legal credibility or admissibility. Second, GC-
and LC-MS/MS (gas- and liquid chromatography/mass spectrometry/mass) tests
have been developed to test for ethanol metabolites in hair and meconium in
order to screen for alcohol abuse during pregnancy.3,4 Finally,
improvements have been made in screening for chronic alcoholism by a
combination of patient information and blood-chemistry analyses. These
assessments do not test directly for ethanol or ethanol metabolites but,
rather, for physiological changes associated with chronic alcohol
consumption. One of these, the early detection of alcohol consumption or
EDAC test, combines several independent lab analyses and can be used to
detect alcohol consumption of at least three to five drinks per day in the
previous four to six weeks. The EDAC test appears to be an improvement over
established alcoholism screening tests including gamma-glutamyltransferase
or GGT and carbohydrate-deficient transferrin or CDT.5,6 It may
be of use in legal cases to monitor compliance or test for abuse.

—Jennifer Hackbarth, PhD

Clinical Chemistry Fellow

Department of

Laboratory Medicine and Pathology

Mayo Clinic

Rochester, MN

References

  1. Karch SB, ed. Forensic Issues in Alcohol
    Testing
    . Florida:CRC Press, 2007.
  2. Dettling A, Fischer F, Bohlers S, Ulrichs F, et
    al. Ethanol elimination rates in men and women in consideration of the
    calculated liver weight. Alcohol. 41(6):415-420.
  3. Karbouche H, Sporkert F, Trovler S, Augsburger M,
    et al. Development and validation of a gas chromatography-negative
    chemical ionization tandem mass spectrometry method for the
    determination of ethyl glucuronide in hair and its application to
    forensic toxicology.
    J Chromatogr B Analyt Technol Biomed Life Sci
    . Epub (ahead of
    print). December 6, 2008.
  4. Morini L, Marchei E, Pellegrini M, Groppi A, et
    al. Liquid chromatography with tandem mass spectrometric detection for
    the measurement of ethyl glucuronide and ethyl sulfate in meconium: new
    biomarkers of gestational ethanol exposure? Ther Drug Monit.
    30(6):725-732.
  5. Harasymiw J, Bean P. The Early Detection of
    Alcohol Consumption (EDAC) test shows better performance than gamma-glutamyltransferase
    (GGT) to detect heavy drinking in a large population of males and
    females. Med Sci Monit. 2007;13(9):PI19-124.
  6. Harasymiw J, et al. Using routine laboratory
    tests to detect heavy drinking in the general population. J Addict
    Dis
    . 2006;25(2):59-63.
Differences between INR instruments

Q Am I incorrect in my understanding that INR is a
standardized measure of prothrombin time, regardless of the testing method
used or the number of seconds measured? I ask because of the following data
reported on samples from a patient:

Day 1 PT = 83.5 sec., INR = 4.8

Day 2 PT = 78.3 sec., INR = 6.7

Two different automated coagulation analyzers in the
same laboratory were used for these results. Can anyone explain this to me?

A The INR was introduced in 1983 by the World Health
Organization (WHO) in an effort to offset variation in thromboplastin
reagent responsiveness and enhance standardization of PT reporting. The
reagent known as thromboplastin is a mixture of tissue factor, phospholipid,
and calcium ions, and is used to initiate clotting as measured in the
prothrombin time (PT) assay. Thromboplastin reagent can be prepared by a
variety of methods including tissue extraction, tissue culture, and
molecular biological (genetic) technologies. Thromboplastins from different
sources and methods of manufacture contain different concentrations and
mixtures of components, and this leads to variation in the responsiveness of
the reagent; that is, difference in the degree of PT prolongation induced by
decreased vitamin K-dependent factor activities. In order to account for PT
reagent responsiveness, an International Sensitivity Index (ISI) is assigned
to each commercial lot number of thromboplastin reagent. The ISI is a
calibration parameter that defines the responsiveness of the reagent
relative to a WHO International Reference Preparation (IRP) which, by
definition, has an ISI of 1.0.

The INR is a mathematical conversion of the PT
calculated as follows: INR = (Plasma PT : MNPT) ISI. The mean
normal PT is the geometric mean of the PT of approximately 20 healthy
individuals drawn via the blood-collection system in use locally and tested
with the same make and lot of thromboplastin as that of the ISI in use.
Though the INR system has improved PT reporting, it is still associated with
unexpectedly high degrees of inconsistency in values between laboratories —
and even within one laboratory between two different instruments
.

An important source of variation in the INR system is
the manufacturer-assigned ISI value compared to the “true” ISI value based
on local calibration of the thromboplastin against an IRP. Variation in ISI
can significantly impact INR, especially when the ISI values are ?2.0. There
are a number of reasons why the ISI value of the thromboplastin reagent in a
particular laboratory on a particular instrument may vary from the “true”
ISI value. Causes for variation in local ISI include but are not limited to
imprecision in the assignment of ISI by the manufacturer, incorrect ISI
value used by the laboratory in the INR calculation, and the local effect of
the coagulation instrument on the ISI. For a more detailed review of
variations in local ISI, the reader is referred to CLSI Document H54
Procedures for Validation of INR and Local Calibration of PT/INR Systems
.

Thromboplastin reagents that are used in the
laboratory are not calibrated directly against the IRP but, rather, are
calibrated against secondary standards held by the manufacturers. These
reagents in fact, may be three or more calibration steps away from the IRP.
With each calibration, a certain amount of imprecision in the ISI value
occurs and is allowed. If a thromboplastin reagent is three steps away from
the IRP, up to 15% variation in ISI value compared to that determined
against the IRP may be seen. This imprecision in the ISI of the
thromboplastin used locally is only one source of variation in the INR
system.

Another cause of variation is that different
coagulation instruments can have marked effects on the ISI of
thromboplastins. ISI values are, thus, instrument- and reagent-specific.
Variation in ISI values between apparently identical coagulation instruments
using the same thromboplastin may occur. ISI values are considered “generic”
if the ISI determined for a thromboplastin is provided for a group of
instruments that use the same general method for endpoint detection, such as
manual, photo-optical, or mechanical methods. This general scheme of
assigning an ISI is problematic because not all instruments within a group
(i.e., not all optical systems) function in the same way. Whenever possible,
laboratories should use thromboplastin reagents with instrument-specific ISI
values, as this improves INR accuracy.

In the reader's question, it is interesting to note
that the PT in seconds Day One was higher than Day Two, yet the INR on Day
One was lower than Day Two. This would suggest that the reagent used Day One
is more responsive than that used on Day Two, meaning it would have a lower
ISI value. If the same reagent is used on both instruments with the same ISI
value however, some other considerations should be taken into account; and
it is paramount that the components of each INR equation be checked. The
reader should make certain that the correct ISI value for each reagent and
each instrument is used in the INR calculation and that the mean normal PT,
or MNPT, has been determined for each model of analyzer and entered
correctly. Do both instruments use the same method of clot detection? Keep
in mind that ISI values vary by method of clot detection. If a
thromboplastin is used on an optical instrument, the same thromboplastin
would likely have a different ISI with a mechanical endpoint detection
system. ISI values may also vary, depending on the particular coagulation
instrument used.

It must also be kept in mind that increased variation
in INR values is seen when the therapeutic range of INR 4.5 is exceeded.
This variability occurs because above this value INR, results become very
sensitive to changes in PT as determined in seconds. Due to the sources of
variation (listed in this paragraph and the paragraph above), INR results
between instruments or between laboratories, especially when the INR is
above 4.5, may vary significantly, even as high as 20%. Another important
consideration to this reader's question is the physiological variation in
the patient's vitamin K-dependent factor levels between Day One and Two, as
this must be taken into account as well.

Means to optimize the INR system as it currently
exists includes using an instrument-specific thromboplastin reagent and
making certain that the proper ISI value for the method of clot detection is
used in the INR calculation. It may also be helpful to use certified plasmas
to perform INR validation and — if necessary — calibration, although such
plasmas are not yet available in the United States as an in vitro
diagnostic, or IVD, product. Certified plasmas are well-characterized
plasmas that have INR values assigned to them in order to verify that the
ISI used locally is correct. If the INR values of the certified plasmas
determined locally vary from the assigned INR values of these plasmas, this
suggests that local INR calibration is necessary or perhaps a different
thromboplastin reagent should be employed. Additional information on INR
validation and calibration can be obtained through the previously mentioned
CLSI Document H54.

—Dorothy M. (Adcock) Funk, MD, FASCP

Medical Director

Esoterix Coagulation

Austin, TX

Further reading

Adcock DM, Duff S. Enhanced standardization of the
international normalized ratio through the use of plasma calibrants: a
concise review. Bl Coagul Fibrinolysis. 2000;11:583-590.

Poller L, Triplett DA, Hirsh J, et al. The value of
plasma calibrants in correcting coagulometer effects on international
normalized ratios. Am J Clin Pathol. 1995;103:358-365.

Clinical and Laboratory Standards Institute.
Procedures for Validation of INR and Local Calibration of PT/INR Systems;
Approved Guideline
. Wayne, PA: Clinical and Laboratory Standards
Institute; CLSI Document H54-A.

Daniel M. Baer, MD, is professor
emeritus of laboratory medicine at Oregon Health and Science University
in Portland, OR, and a member of MLO's editorial advisory board.

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