Summary. The PFA-100® system provides an in-vitro method of assessing primary platelet-related haemostasis by measuring the time (the closure time, or CT) taken for a platelet plug to occlude a microscopic aperture cut into a membrane coated with collagen and either epinephrine or ADP. We used the system to establish normal ranges for CTs in healthy children, adults and neonates. Mean CTs of healthy children were independent of the needle gauge used (21G or 23G) for blood sampling; they were very similar to the mean CTs of healthy adults, but longer than mean CTs of healthy neonates. Although children with haemophilia had normal CTs, the PFA-100 system was found to be potentially useful in screening for von Willebrand disease in children.

Summary. Desmopressin (DDAVP) increases plasma factor VIII coagulant activity (FVIII:C) levels in patients with mild/ moderate haemophilia A. In some subjects, FVIII can be increased to haemostatic levels, thereby avoiding use of factor VIII concentrates. We reviewed our hospital's experience with 62 boys with FVIII:C levels 0AE01-0AE3 IU/ml who had a DDAVP challenge test (i.v. 0AE3 lg/kg) following diagnosis. A therapeutic response was defined as a 1 h post-FVIII:C increase at least twofold over baseline and > 0AE3 IU/ml. Of the total group, 29 (47%) boys responded to DDAVP, all of them with mild haemophilia (baseline FVIII:C ‡ 0AE05 IU/ml), yielding a response rate of 57% in this subgroup. Boys who responded to DDAVP had higher baseline FVIII:C levels (mean ± SEM, 0AE17 ± 0AE01 vs 0AE10 ± 0AE01 IU/ml, P < 0AE01) and were older (5AE2 ± 0AE8 vs 3 ± 0AE4 years, P ¼ 0AE02) than those who failed to do so. The association between DDAVP response and age, however, remains unclear: seven boys who failed the initial challenge test responded to re-challenge after a mean of 6AE3 years (median 4AE9, range 0AE5-12AE5), increasing the response rate in boys with mild haemophilia to 71%. Age and FVIII:C association with DDAVP response are both important in boys with mild/moderate haemophilia A. Absence of response to DDAVP should therefore be confirmed by later re-challenge.

The incidence of acquired vWD in association with Wilms' tumor merits further study through a large prospective trial. Such a trial should include careful family and clinical bleeding histories plus measurement of a platelet count, BT, coagulant FVIII and vWF levels, RCoF activity, and vWF multimer analysis. The response to 1-desamino-8-D-arginine vasopressin (DDAVP) should be tested in all patients with Wilms' tumor and acquired vWD, including patients with a type III profile, before an invasive procedure is performed. Successful use of DDAVP may avoid exposure of affected patients to blood products.

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Effects of inhaled nitric oxide (NO) on human platelet function are controversial. It is uncertain whether intraplatelet cGMP mediates the effect of inhaled NO on platelet function. We investigated the effect of 30 ppm inhaled NO on platelet aggregation and plasma and intraplatelet cGMP in 12 subjects. We performed platelet aggregation studies by using a photooptical aggregometer and five agonists (ADP, collagen, epinephrine, arachidonic acid, and ristocetin). During inhalation, the maximal extent of platelet aggregation decreased by 75% with epinephrine (P < 0.005), 56% with collagen (P < 0.005), and 20% with arachidonic acid (P < 0.05). Responses to ADP (8% P > 0.05) and ristocetin (5% P > 0.05) were unaffected. Platelet aggregation velocity decreased by 64% with collagen (P < 0.005), 60% with epinephrine (P < 0.05), 33% with arachidonic acid (P < 0.05), and 14% with ADP (P > 0.05). Plasma cGMP levels increased from 2.58 +/- 0.43 to 9.99 +/- 5.57 pmol/ml (P < 0.005), intraplatelet cGMP levels were unchanged (means +/- SD: 1.96 +/- 0.58 vs. 2.71 +/- 1.67 pmol/109 platelets; P > 0.05). Inhaled NO inhibits platelet aggregation via a cGMP independent mechanism.

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Platelets stored in CP2D have reduced in vitro function after 5 days of storage, but subsequent centrifugation to reduce the plasma volume does not further alter these platelets.