are a group of disorders caused by problems with platelet granules. Granules are little sacs inside the platelet in which proteins and other chemicals important to its function are stored. There are two types of granules: alpha granules and dense granules.

Some storage pool deficiencies (PSDP) are caused by a lack of granules, but the most common ones are caused by a failure of the platelets to empty the contents of the granules into the bloodstream (this is called the secretory or release mechanism).

There are four major inherited forms of PSDPs: Dense Body Deficiency, Gray Platelet Syndrome, Quebec Platelet Disorder, and Mixed Alpha-Granule/Dense-Body Deficiency. Each form has a wide variety of symptoms, and even within each type, symptoms can be very different.

• Release defects are a diverse group of disorders caused by a problem with the secretory mechanism. Even though the granules are present within the platelets, their contents are not emptied into the bloodstream properly.
• Delta storage pool deficiency is a platelet function disorder caused by a lack of dense granules and the chemicals normally stored inside them. Without these chemicals, platelets are not activated properly and the injured blood vessel does not constrict to help stop bleeding. This type of bleeding problem can be a feature of other inherited conditions (such as Hermansky-Pudlak syndrome and Chediak-Higashi syndrome).
• Grey platelet syndrome is a very rare platelet function disorder caused by a lack of alpha granules and the chemicals normally stored inside them. Without these proteins, platelets cannot stick to the blood vessel wall, clump together the way they should, or repair the injured blood vessel.
• Combined alpha-delta platelet storage pool deficiency is a rare hemorrhagic disorder due to a constitutional platelet anomaly characterised by moderate to severe deficiency in both platelet alpha-granules and dense bodies, resulting in impaired platelet function and decreased aggregation responses. Patients present increased bleeding tendency with symptoms like easy bruising, or menorrhagia.
• Quebec Platelet Disorder (QPD) is an autosomal dominant bleeding disorder associated with reduced platelet counts and a unique gain-of-function defect in fibrinolysis due to increased expression and storage of urokinase plasminogen activator (uPA) by megakaryocytes. QPD increases risks for bleeding and its key clinical feature is delayed-onset bleeding, following surgery, dental procedures or trauma, which responds only to treatment with fibrinolytic inhibitors.