|Prothrombin Time Testing Practices in the Pacific Northwest:
A Model for Monitoring Voluntary Practice Guidelines
Medical errors are unacceptably high despite a long-standing focus on quality
improvement activities. It has been suggested that identifying and monitoring
indicators of quality and adopting standards of practice can reduce medical
errors. One laboratory test that is common and vulnerable to error is the
prothrombin time (PT) test. Prothrombin time testing is performed in almost
all hospitals and is now common in non-hospital settings as well. Problems may
arise when patients move from one test setting to another and when individual
testing sites change the reagent lots they use for PT assay if certain accepted
standards of laboratory practice are not followed.
Purpose of the study
In October 2003, the Washington State Office of Laboratory Quality
Assurance and the Centers for Disease Control and Prevention entered
into a cooperative agreement to study laboratory testing practices in
the Pacific Northwest. Using PT testing as a model, the purpose of
the study was to
- evaluate current testing practices,
- determine which practice recommendations and guidelines laboratories use,
- assess the reasons testing sites did not adhere to accepted standards of practice,
- compare practices in different settings and by different test methods, and
- educate laboratory personnel about the commonly accepted standards of practice.
A questionnaire was mailed in January 2004 to 591 testing sites that performed PT testing in
the Pacific Northwest. Two hundred and ninety-seven returned a completed form, resulting
in a 50% response rate. Various types of settings, locations, test complexities, test
methods and testing personnel were represented in the questionnaire responses.
To determine if testing personnel use voluntary practice standards to develop their
policies and practices, we focused on 4 areas important for PT testing:
- development of specimen collection policies,
- concentration of sodium citrate in specimen collection tubes,
- use of PT testing reagents with low international sensitivity index (ISI) values, and
- validating new lots of PT test reagents.
The use of voluntary practice standards to develop testing policies and practices was
adhered to by a minority of the survey respondents:
Most sites using traditional PT methods were adhering to recommended practice standards. They used standards
recommended by organizations (the CLSI, CAP, WHO, etc.) as well as manufacturer recommendations, journal
articles, government regulations and accreditation standards to develop their policies and practices:
- 46% of sites used a practice standard to develop their written specimen acceptance/rejection policies.
- 39% of sites used a practice standard to select the concentration of the sodium citrate
anticoagulant in their specimen collection tubes.
- 25% of sites using traditional methods and 11% using point-of-care (POC) devices used a practice standard
to select the ISI of their PT reagents.
- 37% of those using traditional methods and 25% of those using POC devices used a practice standard to
develop their policies for validating new lots of test reagents.
- The most common reason why standards were not used was "lack of awareness." For sites using POC devices,
a commonly stated reason was "standards did not apply."
- Among those using voluntary practice standards, those by the Clinical and Laboratory Standards
Institute (CLSI)-formerly known as NCCLS-, College of American Pathologists (CAP), and World Health
Organization (WHO) were the most commonly mentioned in the order of decreasing frequency.
Most practice standards do not apply to PT testing performed with POC devices using capillary specimens.
- 94% of the respondents collecting samples by venipuncture said they had a written policy addressing
specimen acceptability and rejection for PT testing.
- 92% exclusively used collection tubes with 3.2% sodium citrate anticoagulant.
- 76% of the respondents employing a traditional method reported using a reagent with an ISI value of less than 1.70.
- 95% of the respondents established their own patient mean of normal and 92% verified their reference range when
implementing new lots of reagents.
The study also gathered information on
- testing methodologies,
- testing personnel,
- reporting formats,
- reasons for repeating tests,
- general quality assurance practices, and
- competency assessment activities.
Laboratory testing personnel can use this report to
compare their practices against their peers.
Individual laboratories can improve the consistency of their test
results when using new lots of PT reagents by
Agreement among different laboratories may be improved by
- verifying that the ISI is correct for their instrument/reagent combination,
- establishing their own patient mean of normal,
- conducting parallel testing between old and new lots,
- verifying the calculation of the international normalized ratio (INR), and
- alerting clinicians to any changes in testing methodology and procedure that
may affect test results and thus clinical interpretation.
- using reagents with low ISI values,
- using collection tubes with 3.2% sodium citrate,
- reporting INRs,
- knowing how each participants' results compare with those of other sites,
- alerting clinicians of any changes in laboratory results due to different
testing methodology that may affect clinical interpretation, and
- promoting awareness of laboratory practice standards through professional societies,
including those representing physicians, nurses, physician assistants and other medical
staff who commonly perform laboratory testing as a component of their overall health care activities.
This page last reviewed: 3/7/2005