Plasma P-selectin is increased in thrombotic consumptive platelet disorders

P-selectin is a 140-kD protein found in the alpha-granules of platelets and the Weibel-Palade bodies of endothelial cells that on cell activation is expressed on the cell surface and also secreted into the plasma. The secreted form of P-selectin, like plasma P-selectin, differed from platelet membrane P-selectin in that its molecular mass was approximately 3 kD lower under reducing conditions. Both the secreted and plasma forms of P-selectin contained cytoplasmic sequence as determined by Western blot analysis with an affinity-purified rabbit anti-P-selectin cytoplasmic peptide antibody. We have measured plasma P- selectin and beta-thromboglobulin (beta TG) concurrently in (1) patients with consumptive thrombotic disorders, including disseminated intravascular coagulation (DIC), heparin-induced thrombocytopenia (HIT), and thrombotic thrombocytopenic purpura (TTP)/haemolytic uremic syndrome (HUS); (2) patients with idiopathic thrombocytopenic purpura (ITP); and (3) healthy controls. Patients with DIC, HIT, and TTP/HUS, but not ITP, had significantly elevated plasma P-selectin and beta TG levels when compared with their age-matched healthy controls. The increased plasma P-selectin and beta TG in patients with thrombotic disorders were likely to be the result of in vivo platelet and endothelial cell damage or activation. We also found that avoidance of veno-occlusion and other tedious measures customarily taken during blood collection and sample preparation to prevent in vitro platelet activation did not affect plasma P-selectin assay results. In addition, plasma P-selectin levels were not influenced by the presence of renal failure or heparin administration. These results indicate that plasma P- selectin may be a useful new marker for thrombotic diseases.     

Measurements of immature platelets with haematology analysers are of limited value to separate immune thrombocytopenia from bone marrow failure

Detection of immature platelets in the circulation may help to dissect thrombocytopenia due to platelet destruction from bone marrow failure (BMF ). We prospectively tested the predictive value of immature platelets, measured as immature platelet fraction (IPF ) on the XE‐5000 (Sysmex, Kobe, Japan) or percentage of reticulated platelets (rPT ) on the CD Sapphire (Abbott Diagnostics, Santa Clara, CA, USA) to separate immune thrombocytopenia (ITP ) from BMF (leukaemia, myelodysplastic syndrome, aplastic anaemia). We analysed 58 samples of patients with BMF , 47 samples of patients with ITP and 97 controls. Median rPT (CD Sapphire) was increased to 9·0% in ITP and to 10·9% in BMF , compared to 1·9% in controls. Median IPF (XE‐5000) was 16·2% in ITP , 10·2% in BMF and 2·5% in controls. We found an inverse correlation between high fractions of immature platelets and low platelet counts in thrombocytopenic samples regardless of the diagnosis. In conclusion, we observed a broad overlap of immature platelets between ITP and BMF , which may be caused by an accelerated release of immature platelets in any thrombocytopenic state and decreased production in many patients with ITP . Despite this, IPF (XE‐5000) had some power to discriminate ITP from BMF , whereas rPT (CD Sapphire) was of no predictive value.     

Platelet-bound complement (C3) in immune thrombocytopenia

The fixation of complement to the circulating platelet in immune thrombocytopenia was detected by measurement of one of the complement components, C3, on the surface of platelets from patients with idiopathic thrombocytopenic purpura (ITP) and systemic lupus erythematosus (SLE) using the anti-C3 consumption assay. The surface IgG was determined simultaneously using the previously described anti- IgG consumption assay. Washed platelets from normal controls had 3.5 fg (10(-15) g) of C3, or about 11,000 molecules, per platelet, an amount comparable to the IgG (4.1 FG, or 15,000 molecules, per platelet). For most patients with ITP both C3 and IgG were increased on the platelet surface, although for 5 of 16 patients only IgG was increased. Two patients with SLE and thrombocytopenia had an increase in both C3 and Ig, six patients with SLE who were not thrombocytopenic had normal amounts of membrane-bound C3 and IgG. In 5 patients, 3 with ITP and 2 with collagen vascular disease, both surface immunoproteins decreased with successful treatment of the thrombocytopenia.     

Plasma beta-thromboglobulin: differentiation between intravascular and extravascular platelet destruction.

To elucidate the usefulness of beta-thromboglobulin (beta TG) in the differentiation of the mechanism of thrombocytopenia, plasma beta TG concentration was measured in one patient with amegakaryocytic thrombocytopenia, four patients with autoimmune thrombocytopenia (ATP), two patients with thrombotic thrombocytopenia (TTP), and one patient with thrombocytopenia secondary to disseminated intravascular coagulation (DIC). Plasma beta TG was not measurable in amegakaryocytic thrombocytopenia, was normal in ATP, and was increased in TTP and DIC. These data indicate that in thrombocytopenic patients, increased plasma beta TG concentration may result from intravascular platelet consumption with release of platelet constituents in contrast to extravascular platelet destruction by the macrophage-monocyte system.     

Monocyte-platelet interaction in immune and nonimmune thrombocytopenia

Platelets from 24 patients with immune thrombocytopenia resistant to standard therapy (refractory ITP), 35 patients with nonimmune thrombocytopenia (non-ITP), and 32 normal donors were studied in regard to platelet surface-bound IgG (PBIgG) and the ability of these platelets to be bound by human monocytes in vitro (monocyte-platelet rosette assay). Fourteen (58%) of the platelet samples from refractory ITP patients but none (0%) from the non-ITP or control donors had PBIgG greater than 800 molecules IgG/platelet. Seventeen of 24 (71%) of the ITP patients had platelets which demonstrated increased monocyte- platelet rosette formation [rosette index (RI) greater than 2], whereas only four (11%) of the non-ITP patients had such platelets. There was a direct correlation between PBIgG and rosette index for the platelets from resistant ITP patients. There was no correlation of severity of thrombocytopenia with PBIgG or rosette index. Monocyte-platelet interaction in the presence of elevated PBIgG is mediated through the monocyte Fc-receptor. Platelets from five of ten refractory ITP patients with PBIgG less than 800 molecules IgG/platelet had increased rosette formation. Monocyte-platelet interaction in the absence of increased PBIgG may be due to small amounts of platelet surface IgG which are still able to mediate monocyte Fc-receptor interaction or to alternate membrane receptor interaction through the monocyte C3 receptor. Our data underscore the pathophysiologic relevance of monocyte/macrophage-mediated interaction in immune platelet destruction syndromes.     

IVIg treatment increases thrombin activation of platelets and thrombin generation in paediatric patients with immune thrombocytopenia

Clinical manifestations and laboratory parameters of haemostasis were investigated in 23 children with newly diagnosed immune thrombocytopenia (ITP) before and after intravenous immunoglobulin (IVIg) treatment. ITP patients with platelet counts of less than 20 × 10⁹/L and mild bleeding symptoms, graded by a standardized bleeding score (BS), were compared with healthy children with normal platelet counts and children with chemotherapy-related thrombocytopenia. Markers of platelet activation and platelet apoptosis in the absence and presence of platelet activators were analysed by flow cytometry; thrombin generation in plasma was determined. ITP patients at diagnosis presented with increased proportions of platelets expressing CD62P and CD63 and activated caspases, and with decreased thrombin generation. Thrombin-induced activation of platelets was reduced in ITP compared with controls, while increased proportions of platelets with activated caspases were observed. Children with a higher BS had lower proportions of CD62P-expressing platelets compared with those with a lower BS. IVIg treatment increased the number of reticulated platelets, the platelet count to more than 20 × 10⁹/L and improved bleeding in all patients. Decreased thrombin-induced platelet activation, as well as thrombin generation, were ameliorated. Our results indicate that IVIg treatment helps to counteract diminished platelet function and coagulation in children with newly diagnosed ITP.     

Megakaryocyte apoptosis in immune thrombocytopenia

The mechanisms of platelet underproduction in immune thrombocytopenia (ITP) remain unknown. While the number of megakaryocytes is normal or increased in ITP bone marrow, further studies of megakaryocyte integrity are needed. Megakaryocytes are responsible for the production of platelets in the bone marrow, and they are possible targets of immune-mediated injury in ITP. Since the biological process of megakaryocyte apoptosis impacts platelet production, we investigated megakaryocyte DNA fragmentation as a marker of apoptosis from ITP bone marrow biopsies. Archived bone marrow biopsy specimens from ITP patients, bone marrow specimens from controls with normal platelet counts, and bone marrow specimens from thrombocytopenic controls with myelodysplastic syndrome (MDS) were evaluated. Sections were stained with anti-CD61 for megakaryocyte enumeration, and terminal deoxynucleotidyl transferase dUTP nick-end labeling was used as an apoptotic indicator. In ITP patients, megakaryocyte apoptosis was reduced compared to nonthrombocytopenic controls. Megakaryocyte apoptosis was similarly reduced in thrombocytopenic patients with MDS. These results suggest a link between megakaryocyte apoptosis and platelet production.     

The Exchangeable Splenic Platelet Pool Studied with Epinephrine Infusion in Idiopathic Thrombocytopenic Purpura and in Patients with. Splenomegaly

S ummary . The exchangeable splenic platelet pool (ESPP) was studied with epinephrine infusion and platelet labelling with ⁵¹Cr in five healthy students, 10 patients with idiopathic thrombocytopenic purpura (ITP) and 10 patients with splenomegaly. Five of the ITP-patients were studied after splenectomy. Platelet recovery of infused labelled platelets was calculated in all subjects and also in nine healthy volunteers who had been splenectomized for traumatic rupture of the spleen. Spleen size was determined by gamma camera scintigraphy. It was shown that the spleen is the only site of an exchangeable platelet pool in ITP and that this pool was of the same size in ITP-patients as in the normal controls, viz ∼30% of the total body platelet mass.
In patients with splenomegaly the ESPP was larger than that in controls and ITP-patients. A highly significant correlation was found between the ESPP and the spleen volume. In splenectomized and in non-splenectomized ITP-patients platelet recovery was significantly less than in their respective control groups, indicating that a proportion of the labelled platelets was immediately removed from the circulation after infusion into an ITP recipient and that the recovery of labelled platelets cannot be used as a measure of the ESPP in ITP.
It is suggested that the early destruction of platelets may be due to slight damage to the platelets during the labelling procedure. These damaged platelets can survive in a normal recipient, but are destroyed when infused into the'milieu' of an ITP-patient.     

Platelet apoptosis in paediatric immune thrombocytopenia is ameliorated by intravenous immunoglobulin

To evaluate the role of intravenous immunoglobulin (IVIg) in platelet apoptosis in paediatric immune thrombocytopenia, we investigated the platelets of 20 paediatric patients with acute immune thrombocytopenia (ITP), before and after IVIg treatment. Healthy children with platelet counts in the normal range and children with thrombocytopenia due to chemotherapy were enrolled as controls. All ITP patients presented with platelet counts <20 × 10⁹/l and bleeding symptoms. Markers of apoptosis, including activated caspase‐3, ‐8 and ‐9, phosphatidylserine (PS) exposure, mitochondrial inner membrane potential (ΔΨm), as well as platelet‐derived microparticle formation, were analysed by flow cytometry. After IVIg treatment, platelet counts increased to >20 × 10⁹/l in all patients. ITP patients had significantly increased proportions of platelets with activated caspase‐3, ‐8 and ‐9, with PS exposure, and with decreased ΔΨm, and demonstrated increased microparticle formation. Except for ΔΨm, these markers for apoptosis were reduced by IVIg treatment. Platelets of children with thrombocytopenia after chemotherapy also demonstrated increased microparticle formation and decreased ΔΨm, but no activation of caspases 3, 8 and 9 or PS exposure. In conclusion, in acute paediatric ITP, enhanced platelet apoptosis is seen at diagnosis that normalizes after IVIg treatment.     

Different patterns of platelet turnover in chronic idiopathic thrombocytopenic purpura

Platelet turnover, platelet production, platelet mean life span (MLS), platelet count, mean platelet volume (MPV) and platelet-associated antibodies have been examined in 26 patients with chronic idiopathic thrombocytopenic purpura (ITP) and in 1 patient with hypomegakaryocytic thrombocytopenia (HT). 15 ITP patients had normal or increased platelet turnover and platelet production, while 11 had subnormal values despite shortened MLS, while the patient with HT had normal MLS. The differences between the two groups with high and low platelet turnover were statistically significant. No correlation was found between kinetics parameters and bone marrow pattern in a total of 19 patients examined. These data suggest that in some cases of chronic ITP, the pathogenesis of thrombocytopenia can be due not only to the peripheral destruction of platelets, but also to a deficient platelet production by megakaryocytes. Since the number of megakaryocytes in bone marrow slides is not decreased in the low turnover compared with the high turnover group, it is possible that an impaired pattern of megakaryocyte maturation be the cause of the low platelet production in these patients, unlike in the HT patient where megakaryocytes are almost absent.     

The amount of platelet-bound albumin parallels the amount of IgG on washed platelets from patients with immune thrombocytopenia

The biologic relevance of the increased platelet-associated IgG (PAIgG) on platelets from patients with idiopathic thrombocytopenic purpura (ITP) is unclear. Platelets from ITP patients are often larger than normal, and it is possible that the increased IgG is not specific but passively related to platelet size. The measurement of platelet-bound albumin could provide information concerning the specificity of the platelet-bound IgG, since albumin, like IgG, is a plasma protein, but unlike IgG, is not an active participant in immunologic reactions. Albumin is also a normal constituent of platelet membrane, and increased platelet albumin could indicate an increased platelet mass. Platelet-bound albumin, IgG, and total platelet protein were measured on both intact and disrupted platelets from healthy individuals (n = 25) and patients with ITP (n = 21). Platelet IgG and albumin were measured in an immunoradiometric assay using intact antisera and F(ab')2 fragments prepared from the same antisera. There was no relationship between platelet-bound IgG or albumin, and platelet size measured by either platelet protein or platelet volume, (r less than 0.3 for all interactions). In contrast, there was a significant correlation between platelet-bound albumin and platelet-bound IgG (r = 0.7, n = 21, p less than 0.001). Those patients with elevated platelet PAIgG also had elevated platelet albumin, and this relationship was irrespective of the total platelet protein content or mean platelet volume. It is possible that the increased platelet-bound IgG in ITP reflects an increase in platelet surface area or contaminating platelet fragments that are not manifested as an increase in platelet volume or total platelet protein. Alternatively, a platelet membrane abnormality may occur in ITP that results in the uptake of significant amounts of plasma proteins. Either possibility implies that not all of the IgG on platelets from patients with ITP is pathologic IgG.     

Chronic thrombocytopenia of childhood: use of non-invasive methods in clinical evaluation

OBJECTIVES: An unselected group of 21 children with chronic thrombocytopenia was investigated to understand the patients' platelet abnormality better. METHODS: Platelet counts, mean platelet volumes (MPV), membrane glycoproteins and Fcgamma receptor type IIA (FcgammaRIIA) polymorphism H131R, reticulated platelets (% RP), antiplatelet antibodies and plasma thrombopoietin (TPO) were measured. RESULTS: Sixteen patients had idiopathic thrombocytopenic purpura (ITP) (group 1: platelets < 50 x 10(9)/L, n = 6; group 2: 50-99 x 10(9)/L, n = 4; group 3: 100-149 x 10(9)/L, n = 4; group 4: splenectomised patients with normal platelet counts, n = 2). Five patients had familial thrombocytopenia. Six healthy children were studied as a reference. In the 19 thrombocytopenic patients, the platelets were significantly larger and % RP and TPO levels were significantly higher than those in the controls. Increased megakaryocytosis at diagnosis was associated with larger MPV and higher % RP but not with platelet level or TPO. The % RP was remarkably high in all ITP patients of group 1 indicating a brisk production of platelets despite low peripheral count. In all patients with familial thrombocytopenia, TPO was increased suggesting that the syndrome was not because of defective TPO production. The distribution of FcgammaRIIA alleles in the patients was similar to that in the controls. CONCLUSIONS: A combined application of % RP and TPO could be helpful in classifying patients with chronic thrombocytopenia into different categories. The observations may be of value in the clinical evaluation of ITP patients and lead to avoidance of invasive examinations at least in some patients.     

Micromethod for demonstrating increased platelet surface immunoglobulin G: findings in acute, chronic, and human immunodeficiency virus-1-related immunologic thrombocytopenias

A method has been developed for the demonstration of increased platelet surface IgG that uses 1 ml of blood regardless of the platelet count. Platelets are gel filtered to remove plasma and contaminating lymphocytes. They are then reacted with fluorescein-conjugated antihuman IgG and analysed by flow cytometry. Percent positive staining cells vary from 10% to 80% of total cells examined. A platelet antibody index is derived from the product of percent positive staining cells X mean fluorescence intensity of positive staining cells. All patients studied with chronic idiopathic thrombocytopenia purpura (ITP) or human immunodeficiency virus-1 (HIV-1)-related thrombocytopenia had increased platelet surface IgG. Twelve acute children and 11 chronic children had indices averaging 3.5- and 8.9-fold greater than 12 normal children, respectively. Five of 12 children with acute ITP had normal platelet IgG. There was no linear correlation between the platelet antibody index and platelet count. Platelets of patients with acute, chronic, or HIV-1-related ITP displayed autofluorescence. In chronic ITP, the percentage of platelets displaying autofluorescence had a significant negative correlation with the platelet count. This technique will be a valuable diagnostic tool in the pediatric population.     

Evaluation of the immature platelet fraction in the diagnosis and prognosis of childhood immune thrombocytopenia

Rapid assessment of platelet production would distinguish between thrombocytopenia due to decreased platelet production or increased peripheral platelet destruction. We evaluated the value of immature platelet fraction (IPF) in differentiating immune thrombocytopenia (ITP) from thrombocytopenia secondary to bone marrow failure and its potential use as a prognostic marker. Forty-one young patients with ITP were compared with 14 patients with hematological malignancies under chemotherapy, representing a control group with thrombocytopenia due to bone marrow suppression and 30 age- and sex-matched healthy controls. Patients were studied stressing on bleeding manifestations, organomegaly/lymphadenopathy and therapy. Complete blood count including IPF was performed using Sysmex XE-2100. ITP patients were classified into two subgroups: acute ITP with spontaneous resolution within 3 months from diagnosis and chronic ITP that lasted ≥1 year from diagnosis. Median IPF was 11.8% in patients with ITP, 7% in those with hematological malignancy and 3% in the control group (p?<?0.001). ITP patients had significantly higher mean platelet volume (MPV), platelet distribution width (PDW), platelet large cell ratio (P-LCR) and IPF compared with patients with malignancy or healthy controls, while plateletcrit (PCT) was significantly lower in ITP patients than other groups (p?<?0.001). IPF was increased in patients with chronic ITP compared with acute ITP group (p?<?0.001). Patients with active ITP had the highest IPF followed by those in partial remission, while ITP patients in remission had the lowest IPF. IPF was positively correlated to the number of lines of treatment used, MPV, PDW and P-LCR, while negatively correlated to platelet count and PCT among ITP patients (p?<?0.001). Multiple regression analysis showed that platelet count and P-LCR were independently related to IPF. ROC curve analysis revealed that the cut-off value of IPF at 9.4% could be diagnostic for ITP patients with a sensitivity of 88% and a specificity of 85.7%. We suggest that IPF may be a rapid and inexpensive automated marker for etiology of thrombocytopenia and can be integrated as a standard parameter to evaluate the thrombopoietic state of the bone marrow. It may be considered as a potential prognostic marker for the development of chronic ITP.     

Procoagulant profile in patients with immune thrombocytopenia

Despite their low platelet count some immune thrombocytopenia (ITP) patients seldom bleed, indicating the presence of factors to compensate thrombocytopenia. Moreover, ITP patients may have an increased risk for thrombosis. These facts suggest the presence of procoagulant mechanisms that have not been clarified yet. The aim of this study was to identify these possible factors. Moreover, the utility of rotational thromboelastometry (ROTEM^®) to test haemostasis in these patients was also evaluated. Patients with ITP presented a procoagulant profile due to an increased amount of platelet‐ and red cell‐microparticles, an increased resistance to protein C and the formation of a clot more resistant to fibrinolysis due to augmented levels of plasminogen activator inhibitor‐1, which might reflect an endothelial damage/activation in ITP patients. Despite increased maximum clot firmness and reduced lysis, ROTEM^® profiles showed a prolonged clotting time that might rely on the presence of anti‐platelet antibodies as suggested by the increased lagtime in thrombin generation test caused by plasma from ITP patients on platelets from healthy controls. These results indicate the need to individualize therapeutic treatment for ITP patients, considering their procoagulant profile and the presence of concomitant risk factors. Moreover, ROTEM^® appeared to be useful for evaluating haemostasis in ITP patients.     

Complement in immune thrombocytopenia (ITP): The role of complement in refractory ITP

Immune thrombocytopenia (ITP) is a disorder characterized by low platelets due to increased clearance and decreased platelet production. While ITP has been characterized as an acquired disorder of the adaptive immune system, the resulting platelet autoantibodies provide ancillary links to the innate immune system via antibody interaction with the complement system. Most autoantibodies in patients with ITP are of the IgG1 subclass, which can be potent activators of the classical complement pathway. Antibody-coated platelets can initiate complement activation via the classical pathway leading to both direct platelet destruction and enhanced clearance of C3b-coated platelets by complement receptors. Similar autoantibody interactions with bone marrow megakaryocytes can also result in complement injury and ineffective thrombopoiesis. The development of novel therapeutic complement inhibitors has revived interest in the role of complement in autoantibody-mediated disorders, such as ITP. A recent early-phase clinical trial of a classical complement pathway inhibitor has demonstrated efficacy in a subset of ITP patients refractory to conventional immune modulation. In this review, we will analyse the role of complement in refractory ITP.     

Failure of two anti-platelet drugs (indobufen and dipyridamole) to improve thrombocytopenia in liver cirrhosis

The hypothesis that thrombocytopenia in liver cirrhosis (LC) could be due to platelet activation was investigated. 18 patients with thrombocytopenia and LC have been studied. Circulating beta-thromboglobulin (beta TG) was normal, but appeared elevated when referred to platelet count. However, this may not accurately reflect alpha-granule release because of reduced liver cell function. Intraplatelet beta TG, on the contrary, should not be affected by liver cell function. It was markedly depressed, thus truly suggesting the existence of an exhausted state caused by platelet activation. Thromboxane B2 production was slightly increased. This could be due to a compensatory mechanism, or simply to platelet size, which was slightly increased, too. Anti-platelet therapy failed to improve thrombocytopenia. A platelet activation state seems therefore to be present in LC, but seems not to be the only cause of thrombocytopenia. Platelet factor 4 approximated zero, regardless of platelet count and therapy. This confirms that elevated values of such a protein represent only a laboratory artifact, due to platelet activation in vitro.     

Interleukin-7 is decreased and maybe plays a pro-inflammatory function in primary immune thrombocytopenia

Primary immune thrombocytopenia (ITP) is an autoimmune disease with many immune dysfunctions, including over-proliferation and apoptosis resistance of auto-reactive lymphocytes. This study aimed to determine the effects of interleukin (IL)-7 on the cytokine production and survival of peripheral blood mononuclear cells and bone marrow mononuclear cells from ITP patients. We found that the plasma IL-7 levels in peripheral blood from ITP patients were lower than that of the normal controls, and it had positive correlation with platelet counts. However, the levels of IL-7 did not change in bone marrow serum of ITP patients compared with that of normal controls. The result of further stimulation experiments in vitro showed that IL-7 up-regulated the apoptosis of autologous platelets, promoted the proliferation and secretion of interferon-γ, tumor necrosis factor-α as well as IL-10 of lymphocyte both from peripheral blood and bone marrow. As the role of IL-7 in apoptosis-resistance and stimulation of pro-inflammatory cytokines, we speculated that decreased IL-7 in peripheral blood, maybe, is a consequence of the negative feedback of the pro-inflammatory function in ITP patients.     

The interpretation of platelet kinetic studies for the identification of sites of abnormal platelet destruction

The kinetics of platelets labelled with 111In have been studied in a series of 175 subjects including 18 normal volunteers, and 12 patients with idiopathic thrombocytopenic purpura (ITP), but excluding patients in whom there was scintigraphic evidence of intravascular platelet consumption. From analysis of the kinetics, the following parameters were calculated: splenic blood flow (SBF), intrasplenic platelet transit time (t-), splenic platelet pool capacity (expressed as a percentage of the total circulating platelet population), the fraction of the dose of labelled platelets ultimately destroyed in the spleen and the mean platelet life span (MPLS). SBF increased with increasing spleen size up to values of 25% total blood volume (TBV) per min. Some patients with immune complex related diseases were identified with elevated SBF (up to 24% TBV min-1) but without significant splenomegaly. Patients with cardiac decompensation had reduced SBF relative to spleen size. t- showed no relationship with spleen size. It tended to fall in patients who had high SBF relative to spleen size and to rise in those with low SBF relative to spleen size; i.e. it was inversely related to splenic perfusion (flow per unit tissue volume). The splenic platelet pool capacity is dependent on platelet input (SBF) and splenic platelet clearance (reciprocal of t-), and showed a close relationship with spleen size. When all subjects except those with ITP were considered, splenic platelet destruction showed a good correlation (r = 0.70, n = 42, P less than 0.001) with the splenic platelet pooling capacity. The ratio of the fraction of platelets destroyed in the spleen to the fraction pooling there, the D/P ratio, was approximately unity and did not appear to vary with MPLS, spleen size or the patient's condition, except in ITP where it varied between about 0.5 and 2. This variation in ITP was thought to be the result of an immune mediated re-direction of reticulo-endothelial platelet destruction. It is suggested that the D/P ratio, rather than the absolute quantity of 111In labelled platelets destroyed in the spleen, may be a more useful predictor of response to splenectomy since it takes into account the observed, appropriate, tendency for the spleen to destroy platelets in proportion to its platelet pooling capacity.     

Relationship of Raised Platelet IgG in Thrombocytopenia to Total Platelet Protein Content

S ummary . Platelet-associated IgG (PA IgG) of washed platelets of 20 normal controls and 45 thrombocytopenic patients was measured by radioimmunoassay and related to total platelet protein content.
Mean values (±2 SD) for 20 normals were 4.4 ± 3.5 fg IgG per platelet (fg IgG/pl), 2.1 ± 1.0 pg protein per platelet (pg protein/pl) and a ratio of IgG per g platelet protein of 2.1 ± 1.8 mg IgG/g protein. Patients could be divided into two groups: Group I (31) with normal IgG/g protein ratios and either normal (16) or raised (15) fg IgG/pl, and Group II (14) with both fg IgG/pl and IgG/g protein raised. Of the 21 patients with idiopathic thrombocytopenia (ITP), 13 were in Group I (seven with raised and six with normal fg IgG/pl), and eight in Group II. There was significant correlation between PA IgG (fg IgG/pl) and platelet protein for the entire Group I ( r =0.95, P < 0.001). The ITP patients in this group showed a similar correlation ( r =0.77, P < 0.001), suggesting that raised PA IgG levels in Group I patients may be a result of protein-rich platelets.
60-90% of PA IgG was found in the soluble fraction of lysates of platelets from both normals and patients. Thus, it is either non-specifically associated with platelets or is antibody directed towards easily solubilized platelet antigens. Nine thrombocytopenic patients without immune disorders had raised PA IgG. These results question the assumption that raised PA IgG always indicates specific antiplatelet antibody.