Why is the APTT shortened?

Factor VIII is an acute phase reactant. In stress, the levels of FVIII go up, resulting in the APTT shortening.

What is the role of thrombophilia testing in someone presenting with an unprovoked thrombosis?

In acute VTE, aside from life or limb threatening presentations which require consideration of thrombolysis, the standard of care in absence of bleeding contraindication remains anti-coagulation and thrombophilia testing does not alter this management.

Good to read: Does thrombophilia testing help in the clinical management of patients?

What is the role of IVC filter insertion in patients who cannot be anti-coagulated?

Only 3 RCTs of IVC filters have been done. PE was diagnosed with V/Q scan at baseline and between day 8 to 12 for asymptomatic patients. Symptomatic patients had V/Q scans earlier than day 8, when they presented. All patients who had abnormalities on V/Q scan underwent confirmatory pulmonary angiography. Although there was a reduction in confirmed PE at D12 for IVCF group, this was driven by asymptomatic PE and there was no overall survival benefit. This should be considered a negative study form clinical significance perspective. Evidence is then extrapolated to use in VTE patients who cannot be anti-coagulated (Decousus et al., 1998). The other two RCTs are PREPIC and PREPIC 2. Both showed no effect on overall survival. The original PREPIC study reported an increased risk of DVT in the filter group.

Most of the evidence for IVC filter insertion in patients who cannot be anti-coagulated is based on retrospective cohort studies. Patients who have a contraindication to anti-coagulation and acute VTE may benefit from an IVC filter, with a 30-day mortality reduction of 32% (HR 0.68, 95% CI, 0.52-0.88). However, IVC filter use did not reduce the risk of subsequent PE; risk of subsequent DVT increased by 50% in patients with no contraindication to anti-coagulation, and 135% in those who did (White et al., 2016). BCSH guidelines recommend IVC filters only in patients who cannot be anti-coagulated. ACCP and AHA guidelines recommend IVC filters in patients with PE or proximal DVT who have a contraindication to anti-coagulation, or who are actively bleeding. All guidelines agree that IVC filters should be removed within the filter retrieval window when the patient is able to resume anti-coagulation.

What conditions should be considered in unusual venous thrombosis (e.g. multiple or rare sites)?

Cancer, hypercoagulable states, myeloproliferative neoplasms, PNH, anti-phospholipid syndrome.

Good to read: BCSH guideline on thrombosis at unusual sites