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PT and PTT Reference Interval

Here are the results of our latest Quick Question:

How many normal subject specimens do you collect to establish a PT and PTT reference interval?
a. 240: 7 votes, 3%
b. 120: 79 votes, 35%
c. 30: 51 votes, 23%
d. 20: 79 votes, 35%
e. Other: 9 votes, 4%

Comments excerpted from Fritsma GA. Chapter 5, Quality Assorance in Hematology and Hemostasis Testing, Keohane EM, Smith LJ, Walenga JM. Rodak’s Hematology: Clinical Principles and Applications, 2016, Elsevier:

To develop a reference interval, the laboratory professional carefully defines the desired healthy population and recruits representative donors who meet the criteria to provide blood specimens. The definition may, for example, exclude smokers, women taking oral contraceptives, and people using specified over-the-counter or prescription medications. Donors may be paid. There should be an equal number of males and females, and the chosen healthy donors should match the institution’s population demographics in terms of age and race. When practical, large-volume blood specimens are collected, aliquotted, and placed in long-term storage. For instance, plasma aliquots for coagulation reference interval development are stored indefinitely at –70° C. It may be impractical to develop local reference intervals for infants, children, or geriatric populations. In these cases the laboratory director may choose to use published (textbook) intervals. In general, although published reference intervals are available for educational and general discussion purposes, local laboratories must generate their own reference intervals for adults to most closely match the demographics of the area served by their institution.

The minimum number of subject specimens (data points) required to develop a reference interval may be determined using statistical power computations; however, practical limitations prevail. For a new assay with no currently established reference interval, a minimum of 120 data points is necessary. In most cases, however, the assay manufacturer provides a reference interval on the package insert, and the local laboratory practitioner need only assay 30 specimens, approximately 15 male and 15 female, to validate the manufacturer’s reference interval, a process called transference. Likewise, the practitioner may refer to published reference intervals and, once they are locally validated, transfer them to the institution’s report form.

Refer to Clinical and Laboratory Standards Institute (CLSI): Defining, Establishing and Verifying Reference Intervals in the Clinical Laboratory; Approved Guideline—Third Edition. CLSI document EP28-A3c. Wayne, Pa, 2010.

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