In the oral contraceptive example, we inferred that diagnostic suspicion bias was not playing a role because the strength of association did not change when the controls were referred patients. In the case of travel and VT, the association almost completely disappeared with referred controls. Does this finding mean that the association between travel and VT is due to diagnostic suspicion and referral bias? Not necessarily. A pivotal assumption when using referred controls is that the exposure (such as oral contraceptives or travel) does not cause the same signs and symptoms as the disease that defines case status (6). Oral contraceptives are very unlikely to cause acute marked edema by a nonthrombotic mechanism. However, travel, in particular long air travel, does induce edema that is similar to the swelling of leg thrombosis and may lead to referral (7–8). In addition, all studies with referred controls included persons with a history of VT (up to 20% of both case patients and controls), which would make them prone to venous insufficiency (and therefore likely to develop edema after long periods of sitting) and easily alarmed by (and likely to seek care for) leg swelling. Because the diagnosis of recurrent thrombosis is not straightforward, adjudicated case patients and controls may be misdiagnosed, which would also bias the association of travel and VT toward the null.