A new imaging technique that can help distinguish between active and chronic thrombi has the potential to reduce the rates of overdiagnosis of recurrent VTE and subsequent use of anticoagulants.
Magnetic resonance direct resonance imaging (MRDTI) can visualise the oxidation of haemoglobin to methaemoglobin in an active thrombus.
Speaking at the ISTH Congress in Melbourne, Dr Frederikus Klok said current imaging can only indirectly visualise thrombi in the venous system which leads to diagnostic uncertainty.
Dr Klok, from the Leiden University Medical Centre in the Netherlands, said a definitive diagnosis cannot be made in about 30% of patients.
“So if the clot is in the legs or the arms, then we use compression ultrasound and compress the vein. If it can be compressed completely, there is no clot, if the vein cannot be compressed it is a clot. It’s very easy test.”
“For pulmonary artery thrombosis, you cannot do compression ultrasound so there we make use of CT with contrast. Normally the complete pulmonary artery should be filled with contrast. If there is some location within the pulmonary artery tree where there is no contrast, also called a filling defect, then there should be a clot.”
“Both the compression as well as the CT with contrast are indirect measurements because you cannot see the clot. You don’t know if it is old or new.”
“If you do an ultrasound and there is something there, then if you are not certain, these patients tend to get treatment.”
The validation of MRDTI involved a prospective, multicentre study of 305 patients with suspected recurrent ipsilateral DVT of the leg.
At baseline, MRDTI indicated no DVT in 61% of patients. Within a three-month follow-up period, only two patients developed symptomatic VTE. None were fatal.
The failure rate in patients who did not receive any form of prophylactic anticoagulation was 1.65% – well below the predetermined safety threshold of 6.5%.
Importantly, almost half (48%) of patients with a negative MRDTI had an abnormal ultrasound at baseline.
He said about one in five patients were saved from unnecessary anticoagulant use, associated bleeding risk, and health care burden.
“We concluded that MRDTI is a safe and reproducible test for ruling out recurrent DVT. We have shown now that if it is old, and there is no positive signal, you can leave it untreated and it is safe,” Dr Klok said.
However the test should only be reserved for patients where conventional imaging could not make the diagnosis.
It may also prove to be useful in other challenging settings such as pregnancy, he said.