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Effectiveness of the RhIg dose calculator.

To the Editor.--An analysis of responses to the supplemental questions in the College of American Pathologists (CAP) proficiency testing for the Fetal RBC Detection Survey (HBF-A 2007 Survey) revealed that 20.7% of participants who used the method of the AABB Technical Manual (1) to calculate the postpartum dose of Rh immune globulin (RhIg) would have recommended an inaccurate dose. This observation prompted the CAP Transfusion Medicine Resource Committee to develop the RhIg Dose Calculator, a Microsoft (Redmond, Washington) Excel software program that calculates the recommended dose of RhIg according to the method of the AABB Technical Manual. (2,3) The software program requires users to enter the percentage of fetal red blood cells in a sample of maternal blood (determined by a quantitative laboratory assay) and the woman's height and weight (or an assumed 5000-mL blood volume). The RhIg Dose Calculator has been available in the Transfusion Medicine Topic Center of the CAP Web site (2) since 2008.

To evaluate the effectiveness of the RhIg Dose Calculator, supplemental questions were included in the HBF-B 2009 Survey, asking participants to submit the dose (number of vials) of RhIg that their laboratory would recommend for a postpartum Rh(D)-negative woman whose quantitative assay result for fetal RBCs was "1% = 50 mL whole blood." (4) Of 1681 participants responding, only 1263 (75.1%) submitted the correct answer (3 vials), according to the method of the AABB Technical Manual. (4) Of 418 (24.9%) participants submitting an erroneous or "other" response, 187 (44.7%) reported that they used the method of the Technical Manual. Of 1122 participants who calculated the dose of RhIg using the method of the Technical Manual, 187 (16.7%) submitted an erroneous or other" response. In contrast, of the 129 participants using the RhIg Dose Calculator, only 2 (1.6%) submitted erroneous results. Laboratories are encouraged to use the RhIg Dose Calculator, if only to verify their standard method for calculating the postpartum dose of RhIg. The results of this survey demonstrate the potential effectiveness of the RhIg Dose Calculator for preventing inaccurate calculations for the recommended postpartum dose of RhIg. Although laboratories are encouraged to use the RhIg Dose Calculator as a method for ensuring accurate calculations according to the method of the AABB Technical Manual, readers should be aware that other formulae are available and may be used to calculate the dose of RhIg. (1) The RhIg Dose Calculator is provided as a service to laboratories. Its use is voluntary and does not, necessarily, ensure a successful medical outcome.

(1.) Ramsey G; for the College of American Pathologists Transfusion Medicine Resource Committee. Inaccurate doses of Rh immune globulin after Rh-incompatible fetomaternal hemorrhage: survey of laboratory practice. Arch Path Lab Med. 2009;133(3):465-469.

(2.) Paxton A. Bringing new rigor to RhIg calculations. CAP Today. 2008;22(5):1.

(3.) Roback JD, Combs MR, Grossman BJ, Hillyer CD, eds. Technical Manual. 16th ed. Bethesda, MD: AABB; 2008:631-632.

(4.) Sandler SG. 2009 CAP surveys: HBF-B fetal RBC detection: Interlaboratory Comparison Program participant summary. Northfield, IL: College of American Pathologists; 2009.

The author has no relevant financial interest in the products or companies described in this article.

S. Gerald Sandler, MD

Department of Laboratory Medicine

Georgetown University Hospital

Washington, DC 20007
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Title Annotation:Letters to the Editor
Author:Sandler, S. Gerald
Publication:Archives of Pathology & Laboratory Medicine
Article Type:Letter to the editor
Date:Jul 1, 2010
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