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.
- CLSI. Procedures and Devices for the Collection of Diagnostic
Capillary Blood Specimens; Approved Standards-Fifth Edition. CLSI
document H4-A5. Wayne, Pa.; 2004.
- Meites S, Levitt MJ. Skin puncture and blood collection techniques
for infants. Clin Chem. 1979;25: 183-189. 3. Kupke IR, Kather
B, Zeugner S. On the composition of capillary and venous blood serum. Clin Chim Acta. 1981;112: 177-185
Stanley Lo, PhD
Department of Pathology
Children’s Hospital of Wisconsin
Medical College of Wisconsin
Consultant, CAP Chemistry Resource Committee
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.
- Blajchman MA, Goldman M, Freedman JJ, et al. Proceedings of a consensus
conference: Prevention of post-transfusion CMV in the era of universal
Med Rev. 2001;15:1-20.
- Ratko A, Cummings JP, Oberman H, et al. Evidence-based recommendations
for the use of WBC-reduced cellular blood components. Transfusion.
2001; 41: 1310-1319.
- Nichols WG, Price TH, Gooley T, et al. Transfusion-transmitted cytomegalovirus
infection after receipt of leukoreduced blood products. Blood.
- Narvios AB, de Lima M, Shah H, et al. Transfusion of leukoreduced
cellular blood components from cyto mega lo virus-unscreened donors
in allogeneic hematopoietic transplant recipients: analysis of 72 recipients. Bone
Marrow Transplant. 2005;36:499-501.
James P. AuBuchon
Department of Pathology
Dartmouth Hitchcock Medical Center
Chair, Transfusion Medicine Resource Committee