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Rhesus Immune Globulin Dosing in the Obesity Epidemic Era.

To the Editor.--Rhesus immune globulin (RhIG) has been licensed in the United States since 1968 and has dramatically reduced the incidence of Rh (D) alloimmunization from 16% to less than 0.1%. (1) Because as little as 0.1 mL of D+ erythrocytes can result in alloimmunization, (1) adequate dosing of RhIG is essential. This is especially important after documentation of a large fetomaternal hemorrhage confirmed by a quantitative assay either during pregnancy and/or at postpartum. Although 300 [micro]g in each vial is expected to protect from alloimmunization 30 mL of fetal blood, we suggest that an underestimation of the maternal total blood volume (TBV) could adversely affect the final dosing calculation for RhIG given.

A recent College of American Pathologists (CAP) survey suggests that there continues to be an opportunity for laboratories to improve their RhIG dosage calculations. (2) The CAP recommends using the CAP RhIG dose calculator (3) based on the AABB Technical Manual. (1) This calculator and the formula in the most recent edition of that book suggest using an assumed maternal TBV of 5000 mL if the patient's height and weight are not known. (1,3) However, given the current obesity epidemic, that may not be a safe assumption. During the past 20 years, there has been a dramatic increase in the obesity prevalence; currently, about 34.9% of the adults in the United States (78.6 millions) are obese. (4) In our experience, it is not uncommon to encounter pregnant patients weighing more than 100 kg, and many weighing more than 150 kg. Therefore, many patients have TBV significantly greater than 5000 mL. Although there is an extra RhIG vial added at the end of the calculation, we argue that it is not ideal for 5000 mL to be the default TBV. The RhIG dosing might be calculated and administered by midlevel providers or physicians who are not familiar with the TBV calculation. Thus, they may not realize the potential consequences of underdosing RhIG, such as an increased risk of D alloimmunization and severe hemolytic disease of the fetus and newborn in future pregnancies. For example, a woman weighing 127 kg who is 1.68 m tall has a TBV of approximately 6070 mL (using the Nadler formula (5)). If she had a fetomaternal hemorrhage of 0.8%, she should receive 3 RhIG vials (not 2 as shown in the table). (1) Therefore, we suggest that the option of using 5000 mL as the assumed TBV should not be available in the CAP RhIG dose calculator or in the AABB Technical Manual. A maternal height and weight should always be obtained before RhIG dosage calculation. Because many clinicians are using tablets as a tool for dosage calculations, an application for RhIG dosage that requires the input of current height, weight, and percentage of fetomaternal hemorrhage would be useful. Additionally, it has been suggested (6) that even the Nadler formula might not be accurate in TBV determination in obese patients; thus, further research should be conducted to improve the blood volume calculation in this setting. If used, our recommendations can improve the accuracy of RhIG dosing in the midst of the current obesity epidemic and may prevent the resurgence of hemolytic disease of the fetus and newborn from anti-D.

doi: 10.5858/arpa.2014-0605-LE

Huy P. Pham, MD, MPH; Marisa B. Marques, MD; Lance A. Williams III, MD

Department of Pathology, University of Alabama, Birmingham

(1.) Kennedy M, Delaney M, Scrape S. Perinatal issues in transfusion practice. In: Fung M, Grossman B, Hillyer C, Westhoff C, eds. AABB Technical Manual. 18th ed. Bethesda, MD: AABB; 2014:565-566.

(2.) Lockhart E. Fetal RBC detection participant summary. In: CAP Surveys 2014 and Anatomic Pathology Education Programs. Northfield, IL: College of American Pathologists; 2014:6-7.

(3.) Transfusion Medicine Topic Center. RhIG Dose Calculator. College of American Pathologists. transfusionmedicine/ Accessed February 9, 2014.

(4.) Ogden CL, Carroll MD, Kit BK, Flegal KM. Prevalence of childhood and adult obesity in the United States, 2011-2012. JAMA. 2014; 311(8): 806-814.

(5.) Nadler SB, Hidalgo JH, Bloch T. Prediction of blood volume in normal human adults. Surgery. 1962; 51(2):224-232.

(6.) Wheelock L, Li Y, Benoit R, Woods L, Weinstein R. Should an adjusted body weight (ABW) be used for plasma volume calculation in obese patients [abstract 74]? J Clin Apheresis. 2008; 23(1):40.

doi: 10.5858/arpa.2014-0605-LE
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Title Annotation:Letters to the Editor
Author:Pham, Huy P.; Marques, Marisa B.; Williams, Lance A. III
Publication:Archives of Pathology & Laboratory Medicine
Article Type:Letter to the editor
Date:Sep 1, 2015
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