Q. For years, my laboratory has used a thromboplastin reagent with
a high International Sensitivity Index, or ISI, around 1.9. Now I read that
labs should use a more responsive reagent with an ISI of 1.4 or less. How is
this likely to affect my prothrombin time and International Normalized Ratio
values? Will the PT in seconds be longer and the INRs stay close to the same
or will the PTs be somewhat the same and the INRs be lower?
A. The International Normalized Ratio method of reporting
prothrombin time values was introduced in 1983 to standardize laboratory monitoring
of oral anticoagulant therapy. Standardization was needed due to the significant
variation in PT results caused by the marked variability in how different commercial
thromboplastin reagents respond to the vitamin K-dependent clotting factors
(II, VII, X) involved in the extrinsic and common pathways. The INR was developed
to normalize the clotting time values by mathematically correcting for differences
in reagent responsiveness.
Each lot of commercial thromboplastin reagent produced is tested against a
World Health Organization reference plasma, and the relationship is expressed
as the International Sensitivity Index. The lower the ISI, the more responsive
the reagent is to the effects of warfarin therapy and the longer the PTs are
for a given factor concentration. The INR is calculated as the PT ratio (patient
PT divided by the geometric mean normal PT) raised to the power of the ISI.
In an ideal situation, if a sample from an individual on stabilized warfarin
therapy were tested using two thromboplastin reagents with two different ISIs,
the PT would be higher with the more sensitive thromboplastin and lower with
the less sensitive reagent, but the INR values would be equivalent.
The precision of the INR is dependent on the ISI through the following equation:
coefficient of variation of the INR = CV (PT ratio) ¥ ISI. Thromboplastins
with a high ISI value, therefore, may lead to greater imprecision in INR values.
In an effort to improve interlaboratory variation, the 1998 College of American
Pathologists Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy
published a recommendation that laboratories use thromboplastins with an ISI
between 0.9 and 1.7, with a preference for reagents with an ISI toward the lower
end of the scale. Although the effect of using sensitive thromboplastins on
interlaboratory precision is modest, data from CAP interlaboratory proficiency
testing programs generally validate this recommendation.
The use of more sensitive thromboplastins is further supported by how variation
or errors in ISI values influence the INR, especially when ISI values approach
2.0 or greater. For example, given a PT ratio of 2.0, a ±5 percent difference
in ISI in a reagent with an ISI value of 2.0 causes the INR to vary from 3.7
to 4.3, while a reagent having an ISI of 1.0 causes the INR to vary from 1.9
to 2.1 (Fig. 1).
Variation in ISI can occur due to local instrument effect altering the value
of the manufacturer-assigned ISI, or it can reflect an error in assignment of
ISI value due to the inherent imprecision in the manufacturer-assigned ISI compared
to the ISI value determined directly against the WHO reference plasma. Finally,
using thromboplastin reagents with low ISI values results in a wider range of
PT ratios to obtain an INR in the therapeutic range (Fig.
2), thus enhancing patient safety.
Despite introduction of the INR system, significant interlaboratory variation
and inaccuracies persist in the INR. Laboratories can reduce this variation
by using a sensitive thromboplastin, using a thromboplastin with an instrument-specific
ISI value, properly determining the geometric mean normal PT for each lot of
thromboplastin, and ensuring that the INR is calculated properly using the appropriate
ISI for each given reagent lot.
Adcock D, Duff S. Enhanced standardization of the International Normalized Ratio
through the use of plasma calibrants: a concise review. Blood Coagul Fibrinolysis.
2000; 11: 583–590.
Fairweather RB, Ansell J, van den Besselaar AM, et al. College of American
Pathologists Conference XXXI on laboratory monitoring anticoagulant therapy:
laboratory monitoring of oral anticoagulant therapy. Arch Pathol Lab Med.
1998; 122: 768–781.
Hirsh J, Poller L. The international normalized ratio. A guide to understanding
and correcting its problems. Arch Intern Med. 1994;154:282–288.
Loeliger EA, Poller L, Samama M, et al. Questions and answers on prothrombin
time standardisation in oral anticoagulant control. Thromb Haemost.
Dorothy M. Adcock, MD
Q. We use NCCLS standard H3-A4 (June 1998) for the order of draw of
venipuncture collections with multiple tubes. We are starting to change from
glass to plastic tubes, and the manufacturer has advised us that the order of
draw is different for plastic serum tubes. Has the NCCLS updated this standard?
Are there updated standards for those tests that are affected by using plastic
tubes instead of glass?
A. There are no updated NCCLS standards that address the impact
of plastic tubes on the order of draw or the analytes being measured. The order
of draw must be obtained from the manufacturer of the plastic tube collection
system that you use. For example, some manufacturers, like Becton, Dickinson
and Company, coat their plastic serum tubes with silica to activate coagulation.
In this case, the citrated (blue top) tube should be drawn before the plastic
tube to ensure that silica is not inadvertently introduced into the citrated
tube, thereby confounding the result of the prothrombin time/partial thromboplastin
With regard to the effect of plastic tubes on particular analytes, CAP Laboratory
General checklist question GEN.40942 recommends that individual laboratories
determine if specimen containers contribute to analytical interference in the
assays they perform. Labs should make this determination using a combination
of literature review, analysis of manufacturer information, and direct testing.
Manufacturers should provide information to help laboratories make the transition
from glass to plastic containers. However, laboratories are responsible for
deciding which tests are most appropriate for plastic collection tubes.
Robert Makar, MD, PhD
Fellow in Transfusion Medicine
Blood Transfusion Service
Massachusetts General Hospital
Junior Member, CAP
Coagulation Resource Committee
Q. Our very small facility provides care for a large rural area that
runs from the Black Hills to Yellowstone, which means we serve local residents
as well as tourists. We have our share of emergency visits for chest pain and
other ailments, and our physicians are asking for B-type natriuretic peptide
results. What is the most cost-efficient testing for BNP?
A. To provide real-time BNP results in an outpatient clinic setting, especially
for tourists who may need acute care and possibly hospitalization, the only
point-of-care testing option is the Biosite BNP assay. Biosite recently released
its Triage Cardio ProfilER system that provides a 15-minute result turnaround
time for BNP, cardiac troponin I, creatine kinase-MB, and myoglobin on the Triage
MeterPlus testing platform. BNP analysis can also be performed separately. Only
a few drops of whole blood are needed.
Bayer and Roche have larger platforms to measure BNP and N-terminal-proBNP,
respectively, that can also provide turnaround times of less than 15 minutes.
The cost per test is competitive for all three BNP assays.
We use the Biosite BNP assay in our emergency department. In this point-of-care
setting, the test helps us triage patients, manage therapy, and assess risk.
Fred Apple, PhD
Medical Director, Clinical Laboratories
Hennepin County Medical Center
Professor, Laboratory Medicine
University of Minnesota
School of Medicine