LabMed

Scott Syndrome

At a Glance

Scott syndrome is an extremely rare, mild to moderate bleeding disorder in which there is a defective platelet phospholipid membrane support. It is characterized by provoked bleeding, including menorrhagia, epistaxis, trauma induced hematomas, oral bleeding after tooth extractions, and post-partum hemorrhage.

The diagnosis requires a strong clinical suspicion, since all conventional coagulation screening tests are normal. The prevalence of this disorder is unknown since it has only been described since 1973.

After platelets adhere to the damaged endothelium, mediated through von Willebrand factor (VWF), and aggregate, mediated through fibrinogen, the recruited platelets activate and liberate their stored granules. Phosphatidylserine is not translocated to the outer phospholipid membrane in Scott syndrome, leading to reduced support capacity of the tenase and prothrombinase complexes and, thus, reduced overall thrombin generation.

Scott syndrome is not associated with any congenital disorders. Platelets in patients with Scott syndrome have normal adhesion, aggregation, and secretion, but they poorly support the coagulation complexes to form thrombin.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

If there is clinical suspicion for Scott syndrome, first rule out other bleeding disorders prior to verification of this rare diagnosis. A reasonable screening hemostatic evaluation should include the following labs: complete blood count (CBC) with differential, peripheral blood smear, PFA-100 (platelet function analyzer-100), prothrombin time (PT), partial thromboplastin time (PTT), and fibrinogen activity. Direct evaluation for von Willebrand disease (VWD) by VWF antigen, Ristocetin cofactor assay and FVIII activity should be considered up front, especially if there are mucocutaneous bleeding symptoms.

There are no associated cytopenias, and the platelets have a normal morphological appearance under light microscopy. Screening PT/PTT and fibrinogen activity will be normal. The closure time in a PFA-100 will be normal, as will platelet aggregometry and electron microscopy of mounted platelets.

If all of the hemostatic screening labs are normal, then consideration of platelet procoagulant activity is the next step. The screening test of choice would be either a Platelet Factor 3 availability test or an Annexin V binding assay.

Definitive diagnosis requires demonstration of a mutation within the ABC transporter (ABCA1) gene.(Table 1)

Table 1.

Test Results Indicative of the Disorder
Platelet factor 3 availability assay Annexin V binding assay Mutational analysis
Prolonged modified Russell's Viper Venom time Abnormal binding Identification of a mutation within the ABC transporter (ABCA1) gene

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

A thorough drug history is recommended prior to performing a PFA-100 or other platelet studies, since recent NSAID or aspirin use (within the previous 7-8 days) can lead to false-positive results. In addition, there are herbal medications and certain foods (e.g., garlic and fatty foods) that can prolong the closure time mostly in the collagen/epinephrine cartridge.

What Lab Results Are Absolutely Confirmatory?

Definitive diagnosis requires demonstration of a mutation within the ABC transporter (ABCA1) gene.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Platelet coagulant activity assessment is abnormal with other rare platelet disorders, including Quebec disorder and variably in patients with Glanzmann Thrombasthenia and Bernard Soulier syndrome.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Since the PFA-100 and platelet aggregometry are normal with Scott syndrome, over the counter (OTC) drugs and herbal medications will not affect the diagnosis. It is recommended to rule out more common bleeding disorders prior to seeking a diagnosis of Scott syndrome, since there fewer than ten cases have been reported.

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