Indiana Blood Center - Clinical Science Information is a new way to “Ask the Experts” at IBC your transfusion medicine questions. Submit questions via e-mail to the following Indiana Blood Center staff.
Jayanna Slayten, IRL Manager
317-916-5186, jslayten@indianablood.org
Jennifer Rhamy, Vice President of Laboratory Services
317-916-5004, jrhamy@indianablood.org
Dr. Dan Waxman, Chief Medical Officer
317-916-5008, dwaxman@indianablood.org
Common Questions
Q: Are plastic serum tubes with clot activator appropriate and cleared by the FDA for use in the blood bank?
A: Since the advent of the OSHA mandate for safer sharps, clinical laboratories across the country, including Indiana , are converting to plastic tubes for sample collection. These tubes are used in all areas of the clinical lab, including the blood bank. When the manufacturer converts from a glass to a plastic specimen collection tube, the manufacturer must obtain FDA approval of the blood collection tubes for in-vitro diagnostic testing by application. Late in 2003, IBC found that plastic red top serum tubes that contain clot activator from several manufacturers had not been approved by the FDA for blood bank testing.
Since January 2004, thirty percent of samples rejected at the Indiana Blood Center Immunohematology Reference Lab (IBC-IRL) have been due to the rejection of red top serum tubes that contain clot activator. The IBC-IRL completed the investigations using EDTA tubes. The reason that these tubes were rejected include:
- These tubes have not been verified by IBC for use in routine blood bank testing.
- Several manufacturers of these tubes have clearly stated in the package insert, that these tubes are not appropriate for blood bank use.
Q:Should a woman who receives RhIg be monitored with titration studies?
A: Recently, the IBC Reference Lab has received several requests to complete titer studies for patients that have recently received an injection of Rh immune globulin. Physicians that ordered the testing were interviewed by the IBC Medical Director.
The physicians stated that the titer was necessary to assess the effectiveness of the prophylactic Rh immune globulin. The physicians stated that if no titer of anti-D was detectable close to delivery, they may choose to administer additional Rh immune globulin.
What is Rh immune globulin?
Rh immune globulin (RhIG) is a concentration of IgG anti-D derived from human plasma. RhIG was licensed in North America and Europe in 1968. It was developed to prevent Hemolytic Disease of the Newborn (HDN) caused by anti-D. A 1mL dose of RhIG is sufficient to counteract an alloimmunization of 15mL of Rh-positive red blood cells. For the protection against HDN due to anti-D in an Rh-negative mother, the general prophylactic adminstration of RhIG is at 28-30 weeks gestation and again within 72 hours of delivery of a Rh-positive infant. Other indications for administration include after an abortion, ectopic pregnancy, chorionic villus sampling, amniocentesis, antepartum hemorrhage or fetal death.
Q: How are blood banks meeting the requirement from CAP and AABB to assess patients post-delivery for a significant fetal maternal hemorrhage (FMH) to determine RhIg dosage?
A: A full dose of anti-D is 300 µg, 1500 IU, or as indicated by the manufacturer. This is sufficient to act as prophylaxis for up to 15 mL of red blood cells or 30 mL of fetal bleed. RhIG is available as either an intra-muscular (IM) injection or intravenous (IV) infusion.
It is believed, although still the source of some debate, that RhIG works by blocking antigen recognition in the induction phase of primary immunization . Postpartum administration may not be effective if the volume of Rh positive red blood cells exceeds the blocking capacity of the RhIG injection or infusion. The incidence of a fetal bleed is estimated at around 0.25% .
1Perinatal Issues in Transfusion Practice. In AABB Technical Manual 15 th Edition; Bethesda, MD: AABB Press, 2005:545-548.
2Mintz, PD. Rh Immune Globulin. In: Mintz PD ed. Transfusion Therapy: Clinical Principles and Practice. Bethesda , MD : AABB Press, 2005:423-436.
Q. Why would a patient be typed as Rh Negative at one hospital and Rh Positive at another?
Background
Monoclonal Rh typing reagents have become more sensitive. As part of this transition, blood bankers have altered their definition of Rh Positive for purposes of transfusion and Rh immune globulin (RhIg) as the ability to discern between a true Rh positive and qualitative difference in the antigen expression becomes more difficult.
Overview of the expression of the Rh antigen
One gene (RHD) codes for the proteins carrying D expression. In most individuals, the presence or absence of the RHD gene results in expression of the D antigen (Rh positive person) and the lack of expression of the D antigen (Rh negative person). For a small percentage of the population, the inheritance of the RHD gene and the expression of the D antigen may be altered.
Qualitative alteration
Red cells lacking parts of the D antigen complex are termed partial D (sometimes referred to as D variant or D mosaic). Partial D red cells have some epitopes of the D antigen missing or weakly expressed. These altered RhD proteins differ sufficiently from the normal RhD expression to allow for anti-D to be produced.
Quantitative alteration
Some D-positive red cells have all epitopes of D, but they are expressed on the red cell surface differently than the average Rh positive. These individuals have a lower number of D antigens on the red cell surface and do not produce allo-anti-D. These are impossible to discern from the partial or variant D cells by serologic methods.
Changes in D Testing Reagents in the Blood Bank
Understanding the molecular basis of altered expressions of the D antigen has allowed manufacturers to produce more sensitive monoclonal reagents. Not all anti-D reagents detect the same partial or weak expressions of D. Therefore:
- Depending on the testing reagent used by the blood bank and the phases used for testing, the interpretation of the D antigen may be different from one lab to another or from historical records.
- Some blood banks have adopted an approach which assigns D negative status to an individual that demonstrates a weak expression with anti-D reagents. This ensures they will be considered candidates for Rh Immune Globulin, will receive Rh negative red cells and be recorded as Rh negative in the database. Hospitals often do not even test serologically for the weak or partial expression of D by anti-human globulin testing.
- Blood centers testing samples for transfusion will use the most sensitive reagents at the most sensitive phases to ensure that weak or partial D expressions are reported as Rh positive so they will not be infused to an Rh negative patient. This can result in a patient being considered Rh negative who is considered Rh positive for purposes of blood donation.
Considerations for clinical decisions based on the lab findings
|
Expression of D |
Can Produce Anti-D? |
Should Receive RhIg? |
Blood for Transfusion? |
|
D Positive |
No |
No |
D Positive |
|
D Negative |
Yes |
Yes |
D Negative |
|
Partial D |
Yes |
Facility Policy |
Facility Policy |
|
Weak D |
No |
Facility Policy |
Facility Policy |
- When a discrepancy occurs in the reported Rh typing, contact the laboratory. The blood bank manager or pathologist can explain the facility’s policy in regard to partial D and weak D testing.
- While there are no data to demonstrate whether it is beneficial, RhIg prophylaxis may be administered to a partial D or weak D individual to prevent anti-D production. RhIg administration presents no greater risk to these patients than D negative patients. In theory, the Rh Immune Globulin may prevent a partial D patient from forming anti-D.
References: (1) Technical Manual, 15th ed, Bethesda, MD: American Association of Blood Banks, 2005. (2) Ortho Clinical Diagnostics educational website W.I.R.E. (3) Mintz P. Transfusion Therapy, 2nd Edition. Bethesda, MD: American Association of Blood Banks, 2005.