The new pandemic H1N1 flu may cause blood clots and other unusual damage in the lungs and doctors need to be on the lookout, U.S. researchers reported on Thursday.

Two studies published in the American Journal of Roentgenology show the need to check X-rays and CT scans for unusual features, and also point out swine flu can be tricky to diagnose in some of the sickest patients.

H1N1 flu is causing a pandemic, and while it is not particularly deadly, it is sickening many younger adults and older children who usually escape the worst effects of seasonal flu.

"It is therefore essential that clinicians be able to recognize possible cases of pandemic H1N1 influenza in high-risk groups so that they order the appropriate diagnostic tests, begin specific antiviral therapy, and prepare to provide intensive supportive measures as needed," Dr. Daniel Mollura of the National Institutes of Health Clinical Center in Maryland and colleagues wrote.

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One middle-aged man who died was not diagnosed until after death, but unusual findings on his X-rays may be able to help doctors save other, similar patients.

Mollura's team found irregularities called ground-glass opacities in the patient's lungs using a CT scan. Although the patient was severely ill and had a fever, he tested negative for flu and doctors did not treat him for it.

The man died five days after he went into the hospital and the autopsy confirmed he had swine flu. The lung lesions seen on his CT scan matched lung damage done by the virus, Mollura and colleagues said.

In another study in the same journal, CT scans of patients with severe cases of swine flu showed many had pulmonary emboli, which block the arteries in the lungs, a team at the University of Michigan found.

Anticoagulant drugs can break up these clots and save lives.

Dr. Prachi Agarwal and colleagues examined 66 patients diagnosed with H1N1, 14 of them who were in the intensive care unit. All 66 got standard X-rays, which can show if a patient has pneumonia.

They performed enhanced X-rays known as computed tomography or CT scans on 15 of the patients, 10 of them who were in the ICU on ventilators to help them breathe. Five of the ICU patients had the blood clots in the lungs, Agarwal reported

"Our study suggests that patients who are severely ill with H1N1 are also at risk for developing pulmonary emboli, which should be carefully sought for on contrast-enhanced CT scans," Agarwal said in a statement.

"The majority of patients undergoing chest X-rays with H1N1 have normal radiographs (X-rays)," she added. Pulmonary emboli are also not normally seen in flu, she said.

"CT scans proved valuable in identifying those patients at risk of developing more serious complications as a possible result of the H1N1 virus, and for identifying a greater extent of disease than is appreciated on chest radiographs."