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Technical Briefs |
Departments of
1
Clinical Chemistry and
2
Transfusion Medicine, Georg-August-University, Robert-Koch-Strasse 40, 37075 Goettingen, Germany
3
Abbott Diagnostika GmbH, 65205 Wiesbaden-Delkenheim, Germany
aauthor for correspondence: fax 49-551-39-12504, e-mail nahsen{at}gwdg.de
Automated complete blood counts (CBCs) use supplementary warnings (flags) to provide alerts for the existence of hematologic abnormalities (1). We observed on several occasions platelet-count interference by cryoglobulins, where the impedance and optical results were in error to a similar extent and did not trigger a delta alert.
Platelets usually are counted by impedance analysis or by optical procedures (refractive and laser light-scattering properties). The CELL-DYN CD4000 system (Abbott Laboratories) simultaneously provides both and offers a further optional approach using immunoplatelet analysis (2)(3). In the latter method, a fluorescein isothiocyanate-labeled monoclonal antibody (CD61; Clone RUU-PL 7F12, IgG1 subclass) binds the glycoprotein (Gp)IIIa part of the GpIIb/IIIa receptor subunit (3). This glycoprotein is expressed by all human platelets, both resting and activated, but not by erythrocytes or leukocytes. The immunochemical platelet count is largely unaffected by sample interferences and allows determination of platelet counts to <1 x 109/L (4). The analytical performance is virtually identical to that reported for alternative flow cytometric immunomethods (5).
We recently encountered four patients with cryoglobulinemia in whom erroneously high platelet counts were obtained by both optical and impedance platelet counts.
Patient 1 had lymphoplasmacytoid lymphoma. Because the CD4000 analyzer
flagged the presence of variant lymphocytes, a peripheral blood smear
was examined. This confirmed the presence of lymphoma cells and also
revealed thrombocytopenia, whereas the automated impedance and optical
analyses suggested platelet counts of 
355 x
109/L (Table 1
). The result from a chamber (hemacytometry) count (38 x
109/L) also indicated thrombocytopenia, and a
CD4000 CD61 immunoplatelet count (42 x
109/L) further substantiated the morphologic
conclusions. Therefore, both the impedance and optical platelet counts
were overestimated and, because the errors were of similar numeric
magnitude, no CD4000 delta alert was given. Consequently, this
patient’s thrombocytopenia would have been missed had the manual slide
review not been performed for other reasons. We processed these same
samples with other hematology analyzers (Sysmex SE8000, Sysmex K4500,
Sysmex XE2100, and Bayer Technicon H3) and found platelet count
overestimates similar to that of the CD4000.
|
Patient 2 (Table 1
) had Waldenström macroglobulinemia, but at the
time of CBC analysis he was not thrombocytopenic. Both the CD4000
(impedance and optical) and Technicon H3 (optical) instruments
significantly overestimated the platelet count. Two other patients seen
during the past 18 months demonstrated similar patterns of
interference. Both had cryoglobulinemia, one associated with
centroblastic lymphoma (patient 3) and the other with Waldenström
macroglobulinemia (patient 4; see Table 1
). These patients also had
several episodes of thrombocytopenia during their hospital stays for
cytoreductive chemotherapy.
Because of these observations, we wished to develop a review procedure
for recognizing such situations in the absence of a significant
difference between CD4000 impedance and optical platelet counts.
Frequency distributions of platelet size may show increased steepness
or asymmetry in the lower ends of platelet size histograms because of
smaller-sized non-platelet particles such as cryoglobulin. The
Technicon H3 analyzer displayed a typical peak indicative of
cryoglobulinemia (6) in its optical platelet size plot (Fig. 1A
; patient 2), but this was not seen in the CD4000 impedance
histogram (Fig. 1B
). In addition, the CD4000 optical scatter graph
(Fig. 1C
) did not appear unduly abnormal, although there was some
suggestion of a significant accumulation of events at the lower end of
the 7° axis scatter. Thus, atypical CD4000 platelet plots are not
always seen in cryoglobulinemia, although other interferences are
usually detected with high efficiency. For comparison, whereas the
Technicon H3 appeared to provide a more immediate visual indication of
sample cryoglobulins, large platelet count overestimates were still
obtained in most cases.
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Examination of graphic plots is not used in all laboratories, and its efficiency may be limited when workload is high. Without it, operators depend on the presence of other hematologic abnormalities, instrument flags, or clinical information. Because patients with cryoglobulinemia, both at clinical presentation and during treatment, almost always show abnormal erythrocyte or leukocyte results, morphologic review is likely.
When spurious platelet counts are suggested by graphic or morphologic review, a systematic approach to subsequent investigation can be initiated. Initially, warming samples (>30 min at 37 °C) before instrument analysis generally resolves cryoglobulin interference and enables platelet counting by standard optical or impedance methods. When interference persists after rewarming, other causes should be considered.
When doubt remains regarding the true platelet count, the conventional
alternative is manual hemacytometry, but its imprecision is
unacceptably high when the platelet count is <20 x
109/L. The CD4000 CD61 immunoplatelet count,
however, has high precision in thrombocytopenia (4) and,
moreover, is not influenced by the presence of cellular or plasma
interferences (Fig. 1D
). Despite the cost of the test (approximately US
$16 in Germany), immunoplatelet counting should be considered in
situations such as platelet transfusions and therapeutic interventions
with bleeding risk.
Few reports have described platelet count interference by cryoglobulins, although awareness of the potential analytical problems is high. Because the presence of thrombocytopenia was masked in two of the four cases described here, we believe that cryoglobulin interference is an important concern for the clinical laboratory.
Acknowledgments
We thank Dr. Hans Seeger for helpful discussions and Prof. Victor W. Armstrong for valuable editorial support.
References
The following articles in journals at HighWire Press have cited this article:
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  Z. K. Shihabi Cryoglobulins: An Important but Neglected Clinical Test Ann. Clin. Lab. Sci., January 1, 2006; 36(4): 395 - 408. [Abstract] [Full Text] [PDF]
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