Q & A
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February 2006 Q: Are finger-sticks or capillary blood-draw tubes appropriate for all chemistries or for general chemistry, therapeutic drug monitoring, or oncology? Have the manufacturers or literature noted any validations? A. The blood volume used for laboratory testing in the pediatric setting is monitored very carefully. Because limited volumes of blood are requested, phlebotomists pay careful attention when collecting capillary blood specimens. Pediatric patients benefit from this type of collection as an alternative to venipuncture, which may be difficult and potentially hazardous. Blood specimens collected via skin puncture are also useful in the adult setting, especially in point-of-care testing and glucose self-monitoring.1,2 The major concerns with puncturing the skin to collect blood deal with contamination from interstitial fluid, intercellular fluid, hemolysis, and residual contaminate from the surface of the skin. Comparisons between capillary and venous serum show lower capillary values for bilirubin, calcium, chloride, sodium, and total protein, and higher capillary values for glucose and potassium.3 Using capillary specimens for PO2 is controversial; however, an arterial specimen is preferred for blood gas analysis. A number of studies investigating the use of capillary blood specimens, conducted more than 25 years ago, demonstrated small differences between basic chemistries.2 Unfortunately, recent studies in the literature investigating the use of skin puncture to collect blood involve a limited number of analytes. References
Stanley Lo, PhD Q: What is the best way to ensure that patients receive the specialized blood components they need? Should the ordering physician be expected to remember to request the correct component? A. This question most often arises with the provision of cytomegalovirus-safe components or the use of leukoreduction or irradiation to avoid potential unwanted effects of transfusion, such as alloimmunization to human leukocyte antigens or post-transfusion graft versus host disease, respectively. Those at highest risk for CMV infection, and thus who should receive CMV-safe components,1 include seronegative pregnant women and their neonates who weigh less than 1,250 grams. Also at high risk for CMV infection are allogeneic hematopoietic stem cell, or HSC, transplant recipients who are seronegative and have a seronegative donor. Seronegative recipients of solid organ transplants from seronegative donors may also benefit because they subsequently receive immunosuppressive therapy. Similarly, HIV-infected individuals can suffer severe consequences from CMV infection. In many facilities, leukoreduced cellular components are regarded as CMV-safe.2 HSC transplant patients would already likely be receiving leukoreduced cellular components because the blood supplier converted the local blood supply to leukoreduced or because the facility recognizes the importance of using leukoreduced components to reduce alloimmunization and platelet refractoriness in these patients. Whether CMV seronegativity should be required as an additional layer of safety is still being debated. Data presented at the 2000 Canadian Consensus Conference on this issue suggested that transmission through CMV-untested leukoreduced components was a rare event, if it occurs at all.1 (The conference recommended that HSC transplant recipients receive leukoreduced and CMV-seronegative cellular components.) A retrospective review of CMV outcomes from the University of Washington, however, suggested that CMV-untested leukoreduced red blood cells—but not leukoreduced platelet units—could transmit CMV.3 Yet in a more recent analysis at MD Anderson Cancer Center, 72 HSC transplant recipients who received just leukoreduced components did not have evidence of CMV morbidity.4 The most appropriate approach for this vulnerable population of patients remains uncertain. Similar dilemmas arise when determining whether patients should receive leukoreduced or irradiated components, or both. Universal application of either restriction may ensure uniformity, but at a financial cost. It may also lead to reductions in efficacy (from cellular loss through leukoadsorption filtration or induction of red cell membrane lesions through gamma irradiation). One should consider all implications before deciding to use either method for all patients. The question remains about how best to capture the information regarding a patient's special component requirements. Some institutions rely on the ordering physician to recognize the need and to include a request for CMV-safe or irradiated components in the first order or possibly every order for transfusion. Once the requirement has been identified, entering it into the laboratory information system may help prevent the inadvertent release of inappropriate components should future orders fail to include the requirement. Some laboratories and institutions take this approach a step further and designate all patients in a particular ward, such as the hematology/oncology ward or the intensive care nursery, to receive special types of components to avoid the complication of attempting to determine patients’ diagnoses or weight, for example. This approach increases the certainty of correct treatment but at the cost of supplying specially prepared components for some patients who may not benefit from them. Using such an approach, patients with lymphoma might receive CMV-safe components by virtue of their residence on a particular ward or because of care by a hematologist, although their diagnoses would not necessarily place them at higher risk for CMV-related morbidity or mortality. Transfusion services that supply components to remote locations face a special challenge. For example, they may not have access to the patient’s location within the transfusing hospital nor to the patient's medical record to determine the diagnosis or CMV serostatus, so they must depend on the physician’s order. The laboratory and the clinicians it serves must recognize that such a system has the potential to omit special requirements. References
James P. AuBuchon |
