QJM Advance Access originally published online on April 8, 2008
QJM 2008 101(7):549-555; doi:10.1093/qjmed/hcn047
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Reticulated platelets as a screening test to identify thrombocytopenia aetiology
From the 1Internal Medicine Department, 2Immunology Laboratory, and 3Hematology Service, Hospital de Sabadell, Institut Universitari Parc Tauli—UAB, Sabadell, Barcelona, Spain
Address correspondence to M. Monteagudo, Internal Medicine Department, Hospital de Sabadell, Parc Taulí S/N, 08208 Sabadell, Barcelona, Spain. email: MMonteagudo{at}tauli.cat
Received 9 October 2007 and in revised form 11 March 2008
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Summary |
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Background: Thrombocytopenia is a common haematological abnormality and no simple diagnostic test is available to diagnose thrombocytopenia pathogenesis.
Aim: To evaluate sensitivity and specificity of reticulated platelets (RP) as a diagnostic test for thrombocytopenia with increased thrombopoietic activity.
Design: Prospective observational study in thrombocytopenic patients.
Methods: A direct, whole-blood, dual-labelling flow cytometric method was used. Direct, whole-blood double coverage was achieved using a monoclonal anti-glycoprotein (GP)-III antibody (CD61 PerCP®) for platelet identification and thiazole orange (Retic-count®) as platelet mARN stain.
Results: RP were measured in 101 thrombocytopenic patients and 104 non-thrombocytopenic controls. The mean RP percentage in 60 thrombocytopenic patients with no increased thrombopoietic activity was 7.5% (CI for 95%: 5.2–9.7) and RP absolute number was 3.2 x 109/l (CI for 95%: 2.1–4.3). The mean RP percentage in 41 thrombocytopenic patients with increased thrombopoietic activity was 30.3% (CI for 95%: 25.1–35.5) and RP absolute number was 6.2 (CI for 95%: 4.8–7.7). The RP percentage cut-off for a diagnosis of thrombocytopenia with increased thrombopoietic activity was 11% [sensitivity 93%, specificity 85%, positive predictive value (PPV) 83%, negative predictive value (NPV) 95%].
Conclusions: RP measurement by flow cytometry, directly from whole-blood, is a useful screening test to differentiate between thrombocytopenia with high or low thrombopoietic activity. A RP percentage in excess of 11%, has a high sensitivity and good specificity for a diagnosis of thrombocytopenia with increased thrombopoietic activity.
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Introduction |
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Thrombocytopenia is a common haematological abnormality and sometimes associated with severe bleeding. A number of causes may lead to low platelet count, but two major mechanisms are implicated in the pathogenesis, namely increased peripheral platelet destruction and decreased bone marrow production. No simple diagnostic test is available to diagnose thrombocytopenia pathogenesis. Examination of the bone marrow megakaryocytic pool is often required in order to quantify thrombocytopoiesis. Bone marrow is not usually examined in thrombocytopenic patients with suggested accelerated peripheral platelet consumption, because it is an invasive and discomforting procedure. Moreover, it is unsuitable for frequent follow-up in thrombocytopenic patients. In this regard, a sensitive and non-invasive test, capable of evaluating the thrombocytopoietic activity, would be of substantial clinical value. Quantifying RNA platelet content by flow cytometry has been proposed for evaluating platelet turnover. Reticulated platelets (RP) are the youngest circulating platelet population that contain rough endoplasmic reticulum and mRNA, and are thus able to synthesize small amounts of protein. RP first description was reported in 19691 by direct visualization in peripheral blood, using a new methylene blue dye. In 1990, Kienast and Schmitz2 published the first report on the diagnostic value of flow cytometric analysis of RP using the thiazole orange (TO) fluorescent dye in patients with thrombocytopenia. RP measurement by flow cytometry is not a standardized technique, because several dyes for mRNA stain, several substrates for RP determination and different incubation times have been described. Even so, all reports have suggested that analysis of RP provides a good estimate of the rate of platelet production in bone marrow.3–5 Recently, simple protocols using whole-blood for RP determination have been reported6,7 with good results. The aim of the present study was to assess prospectively the diagnostic usefulness of RP determined by flow cytometry, directly from whole-blood in thrombocytopenic patients and to evaluate sensitivity and specificity of RP, as a diagnostic test for thrombocytopenia caused by acceleration of peripheral consumption.
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Materials and methods |
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Patients
From June 2002 to June 2005, we prospectively enrolled both admitted and out patients being attended to at Parc Tauli Hospital in Sabadell (Barcelona, Spain) with thrombocytopenia who met with the inclusion criterion. Inclusion criterion: patients with platelet counts fewer than 100 x 109/l, confirmed after peripheral blood film review, could be included in one defined aetiologic group. Exclusion criterion: pseudothrombocytopenia after peripheral blood film review. Also excluded were patients who could not be included in one defined aetiologic group.
Controls and thrombocytopenic groups were defined as follow:
Group A—thrombocytopenia with normal or decreased thrombopoietic activity
- Central thrombocytopenia when there was diminished or defective platelet production. Diagnoses included in this group were: acute or chronic lymphoproliferative disorders, multiple myeloma, acute leukaemia, aplastic anaemia, myelodysplastic disorders and medullar infiltration by solid tumours, based on blood results, bone marrow findings and cytogenetic analysis. Post-chemotherapy thrombocytopenia was defined as thrombocytopenia after chemotherapy treatment that persisted for >7 days after RP determination. No platelet transfusion or infection was present when RP determination was done. All patients in this group remained thrombocytopenics for >10 days after RP determination.
- Thrombocytopenia by abnormal distribution when there was splenomegaly and hypersplenism analytical data. Diagnoses included in this group were liver cirrhosis and other diseases with portal venous system hypertension.
Group B—thrombocytopenia with increased thrombopoietic activity
- Thrombocytopenia due to enhanced peripheral platelet destruction by an immunologic mechanism. This included acute idiopathic thrombocytopenic purpura (ITP), a diagnosis that was made on the basis of clinical history, physical examination, complete blood count and clinical course after treatment. All patients with ITP, fulfilled the criteria indicated by the American Society of Hematology Practice Guidelines.8
- Thrombocytopenia due to enhanced peripheral platelet destruction by non-immunological mechanism. Diagnoses included were disseminated intravascular coagulation (DIC), idiopathic thrombotic thrombocytopenic purpura and haemolytic-uremic syndrome, all defined by classical clinical symptoms and confirmed through blood studies.
Group C—control groups
- Normal control group. Those with a normal platelet count in blood sample.
- Thrombocytosis control group. Those with platelets above 450 x 109/l. They were reactive thrombocytosis. They were a control group to clarify that RP percentage was not correlated with the platelet count.
Methods
RP were identified following the previously described technique by Robinson et al.,5 without sample manipulation, avoiding fixation and blood centrifugation.
Blood collection
Blood samples were collected in haematological tubes (Vacutainer® type, Madrid, Spain; Becton Dickinson, Madrid, Spain) containing ethylene diamine tetra acetic acid (EDTA)-K2 (1.8 mg/ml). Complete blood count was measured using an ADVIA 120 autoanalyzer (Bayer, Madrid, Spain), and thrombocytopenic samples were confirmed after peripheral blood film reviewal. All blood samples were kept at room temperature, until analysis was performed within than 6 h after collection.
Reticulated platelet quantification
Some 5 µl of whole-blood were incubated with 5 µl of PerCP®-labelled anti-glycoprotein III monoclonal antibody (CD61 PerCP® Becton Dickinson, SA) and 30 µl of phosphate-buffered saline (PBS) for 15 min in the dark, at room temperature. A control tube was used for each sample with 5 µl of isotypic mouse control (IgG1-mouse PerCP® Becton Dickinson, SA). After incubation, 1 ml TO (Retic-count®, Becton Dickinson, SA) at 1/10 dilution in Isoton II® was added to the test tube and 1 ml Isoton II® (Beckman-Coulter) solution was added to the control tube. After incubation for 1 h in the dark at room temperature, they were immediately read in the cytometre (FACSCalibur, Becton Dickinson, SA). Platelets were identified by their logarithmic side scatter (SSC) and CD61 positivity expression. Analysis was performed with computer software (CellQuest Pro, Becton Dickinson, SA). A dot plot cytogram (CD61-PerCP® versus TO fluorescence) was generated, and RP rate was expressed as a percentage of TO and CD61-PerCP® double-positive cells in 10 000 identified platelets. The threshold of TO fluorescence was established, the level was >99% of CD61-PerCP® positive population was negative for TO in the control tube. In each session a sample with a normal number of platelets was used as a control.
Reproducibility of the method
Reproducibility was analysed with five blood samples obtained from two normal subjects (low levels of RP) and three thrombocytopenic patients (one with high levels of RP and two with mid-values) by processing 10 replicates, each sample was prepared 10 times. It was expressed as the coefficient of variation (CV).
Statistical analysis
RP percentage and total RP number were determined in all samples. Results were expressed as mean and 95% mean confidence interval (CI), unless differently indicated. Differences between groups were compared by means of the Student's t-test. A P-value equal or lesser than 0.05 was considered statistically significant. To determine the accuracy of the RP percentage in discriminating thrombocytopenia with increased thrombopoietic activity from thrombocytopenia with normal or decreased thrombopoietic activity, a receiver operating characteristic (ROC) curve was made. The cut-off value of RP percentage with the best sensibility and specificity to diagnose thrombocytopenia with increased thrombopoietic activity, was determined. SPSS 13 for Windows computer software was used for the statistical analysis.
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Performance of the method and normal values
The reproducibility of the assay was good, with CVs ranging from 5.9% to 14.3%. Low levels of RP percentage showed higher CVs (13.6–14.3%) than high levels showed (CV = 6.2%) (Table 1). To define the normal range of RP values, RP were determined in 104 controls. In this group, 82 subjects had a normal platelet number and 22 had thrombocytosis. RP percentage in the control group with normal platelet number was 1.3% (CI for 95%: 1.1–1.5) and in the thrombocytosis group was 1.9% (CI for 95%: 1.2–2.4).
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Clinical diagnosis of patients with thrombocytopenia
RP were determined in 101 thrombocytopenic patients. There were 60 patients in the group A, 45 had central thrombocytopenia (11 myelodysplastic disorders, 10 medullar infiltration by solid tumour or lymphoproliferative disorders, 8 post-chemotherapy, 5 acute leukaemia, 4 multiple myeloma, 3 vitamin B12 deficiency, 2 aplastic anaemia, 2 chronic leukaemia) and 15 had thrombocytopenia by abnormal distribution (14 liver cirrhosis, 1 idiopathic portal hypertension). In group B, there were 41 patients, 29 had ITP and 12 had thrombocytopenia due to enhanced peripheral platelet destruction by non-immunologic mechanism (11 DIC and 1 haemolytic-uremic syndrome) (Table 3). Sex and age for each group in which RP were determined is shown in Table 2. Mean of RP percentage in group A (thrombocytopenia with normal or decreased thrombopoietic activity) was 7.5% (IC for 95%: 5.2–9.7%) and mean RP absolute number was 3.2 x 109/l (CI for 95%: 2.1–4.3 x 109/l). Mean of RP percentage in group B (thrombocytopenia with increased thrombopoietic activity) was 30.3% (CI for 95%: 25.1–35.5%) and mean RP absolute number was 6.2 x 109/l (CI for 95%: 4.8–7.6 x 109/l). Eight patients in group A had very high RP percentage (Figure 1), there were three myelodysplastic disorders with excess blasts (open circles), two acute myelogenous leukaemia (asterisk), one myelomonocytic chronic leukaemia (asterisk), one B-lymphoproliferative disorder (asterisk) and one multiple myeloma (asterisk). Comparative analysis between both thrombocytopenic groups shows statistically significant differences in RP percentage (P < 0.0001) and RP absolute number (P < 0.001). Means of peripheral platelets in each group, RP percentage and RP absolute number with 95% CI intervals are summarized in Table 4. The mean of RP percentage and RP absolute number in central thrombocytopenic group, after the exclusion of eight patients with very high percentage, were 5.4% (CI for 95%: 4.1–6.8) and 2.01 x 109 (CI for 95%: 1.6–2.3), respectively. An overall measure of assay performance in discriminating between both groups of thrombocytopenia is the ROC curve, shown in Figure 2. The area under the curve (AUC) is 0.92 (CI for 95%: 0.85–0.98) with SE 0.031. The RP percentage with the best sensitivity and specificity for a diagnosis of thrombocytopenia with increased thrombopoietic activity was 11%; sensitivity 93%, specificity 85%, positive predictive value (PPV) 83%, negative predictive value (NPV) 95%.
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Discussion |
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We measured RP by flow cytometry directly using whole-blood, avoiding extramanipulation, with a dual-labelling method for platelet identification. Our results showed that RP determination is a good non-invasive test for differential diagnosis between thrombocytopenia with increased thrombopoietic activity and thrombocytopenia with normal or decreased thrombopoietic activity. A RP value >11.08% by this simple technique has a good sensitivity and specificity for diagnosis of thrombocytopenia with increased thrombopoietic activity. Although, generally, RP values are related to the technique used for its measurement, our results in non-thrombocytopenic controls are practically identical to values reported, using a different flow cytometry method.9 Regardless of RP values, several report results, agree that RP are a good platelet turnover marker,10–13 including our previous report.14 Recently, Kuwana et al. reported that RP measured from platelet-rich plasma is one of the six initial laboratory findings that discriminated ITP from other diagnoses in isolated thrombocytopenia.15 It seems that RP determination is a reliable measure to predict imminent platelet recovery in thrombocytopenic patients, after intensive chemotherapy.16–18 To date, RP are not routinely used for thrombopoietic evaluation, despite their potential clinical usefulness, because of their technical complexity, variability and lack of standardization. There are some limitations in this technique, because TO labelling of platelets is not specific. Mitochondrial DNA, dense granule nucleotides and other platelet compartments may non-specifically be labelled with fluorescent dyes like TO.19–21 This non-specifically labelling may explain our false positive results in patients with acute leukaemia and myelodisplastic disorders with central thrombocytopenia, most of them with very high count of peripheral white blood cells. In spite of the limitation, this simple technique may be of clinical usefulness as an initial screening test. RP allow the quantitative assessment of reduced production or increased destruction of platelets in thrombocytopenic patients. Some recent reports measure RP, namely immature platelet fraction (IPF) or high fluorescence platelet fraction, directly from whole-blood and using a completely automatically method.22–27 Values of IPF were very similar to our percentage of RP in several thrombocytopenic groups and a IPF value of 7.7% was reported as the best point for highest sensitivity and specificity to discriminate between normal/suppressed group and increased group.28 This data confirms that RP may be measured easily from whole-blood using some flow cytometric method and they may be used as the first step in thrombocytopenic patient study, because automatically RP counting is now available with some haematological analysers.
In summary, RP determination is a reliable marker of platelet turnover and may be as good as an initial screening test for thrombocytopenic patients, as reticulocytes are in studies of anaemic patients. In thrombocytopenic patients, a clinical evaluation along with a high RP percentage may allow to diagnose thrombocytopenia by increased thrombopoietic activity.
Conflict of interest: None declared.
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References |
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