(sVCAM, sICAM, CRP), and manual methods were used on
the other assayed selectins. A few measurements do not fit
any discernable pattern, notably plasma sVCAM-1 on day 3
at 4 and 21 °C; plasma sE-selectin on day 1 at 4 °C and day
3 at all temperatures, whole blood sE-selectin at 4 °C on day
1 and after freeze-thaw cycle 4; and plasma CRP on day 1 at
4 °C and after freeze-thaw cycle 3. These aberrant values
were likely analytic artifacts, because removal of values ⬎4
SDs from the mean or adjustment for run-to-run variability
did not change the results qualitatively. The large CRP
difference observed after freeze-thaw cycle 3 but not after
cycles 4 or 5 suggests assay error or possibly CRP release
from LDL and complement factors. It has been demon-
strated that a portion of systemic CRP is bound to choles-
terol in modified LDL particles (17) and to different com-
plement factors (18). Interaction of CRP with cholesterol and
complement factors might hamper the detection of CRP in
the assay used. No data were available for sE-selectin, but it
is possible that some sE-selectin is bound to different circu-
lating leukocyte types or microvesicles derived from endo-
thelial cells. Freeze-thaw cycles might release these bound
forms of sE-selectin, leading to an increase in detectable
Possible effects of residual platelets in the plasma on
marker values obtained are not addressed in our study.
sVCAM-1, sICAM-1, and sE-selectin, however, are de-
rived mainly from endothelial cells and not from platelets.
Although the presence of platelets in EDTA plasma might
contribute to the absolute amounts of these markers, it
does not influence their stability. In contrast, sP-selectin is
mainly derived from platelets, a characteristic that may
explain the variance observed in sP-selectin at different
time and temperature points.
Our results show that sP-selectin is unstable under all
storage conditions and requires immediate assay. The
other cardiovascular risk markers evaluated were stable
when stored in whole blood samples for several days at
room temperature. sVCAM-1, sICAM-1, and CRP were
stable at 4 and 21 °C in plasma or whole blood for 5 days
and sE-selectin for 2 days. These findings have important
implications for clinical studies measuring these markers,
reducing the need for immediate transfer of samples to a
laboratory for processing and analysis.
Grant/funding support: This study was funded jointly by the
University of Oxford’s Division of Public Health and Primary
Health Care and Diabetes Trials Unit.
Financial disclosures: None declared.
Acknowledgments: We thank Dr. Brian Shine for assistance
with sensitivity analyses of run-to-run variability and quality
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Previously published online at DOI: 10.1373/clinchem.2006.076380
Genomic Profiling of Circulating Plasma RNA for the
Analysis of Cancer, Manuel Collado,
Luis A. Lo´pez Ferna´ndez,
and Manuel Serrano
Spanish National Cancer
Research Centre (CNIO), Madrid, Spain;
Oncology, Hospital Universitario Puerta de Hierro, Ma-
Department of Pharmacogenetics and Phar-
macogenomics, Hospital Universitario Gregorio Mara-
n˜o´n, Madrid, Spain;
National Centre of Biotechnology
(CNB-CSIC), Campus Universidad Auto´ noma, Madrid,
Spain; * address correspondence to this author at: Spanish
National Cancer Research Centre (CNIO), 3 Melchor
Ferna´ndez Almagro St., 28029 Madrid, Spain; fax 34-91-
732-8028, e-mail email@example.com)
Background: The blood of cancer patients is known to
contain fragments of RNA released from the tumor. The
1860 Technical Briefs