is more commonly used. if positive d-dimer, obtain mdct and base treatment on results. high clinical probability. proceed to mdct. if positive, treat, if negative, more tests are needed to exclude pe. a d-dimer of less than 750 ug/l does not rule out pe in those who are at high risk. pulmonary embolism rule-out criteria the pulmonary embolism rule-out criteria (perc) helps assess people in whom pulmonary embolism is suspected, but unlikely. unlike the wells score and geneva score, which are clinical prediction rules intended to risk stratify people with suspected pe, the perc rule is designed to rule out the risk of pe in people when the physician has already stratified them into a low-risk category. people in this low risk category without any of these criteria may undergo no further testing for pe: low oxygen saturations – sao2 <95%, unilateral leg swelling, coughing up blood, prior dvt or pe, recent surgery or trauma, age >50, hormone use, fast heart rate. the rationale behind this decision is that further testing (specifically ct angiogram of the chest) may cause more harm (from radiation exposure and contrast dye) than the risk of pe. the perc rule has a sensitivity of 97.4% and specificity of 21.9% with a false negative rate of 1.0% (16/1666). blood tests in people with a low or moderate suspicion of pe, a normal d-dimer level (shown in a blood test) is enough to exclude the possibility of thrombotic pe, with a three-month risk of thromboembolic events being 0.14%. d-dimer is highly sensitive but not specific (specificity around 50%). in other words, a positive d-dimer is not synonymous with pe, but a negative d-dimer is, with a good degree of certainty, an indication of absence of a pe. a low pretest probability is also valuable in ruling out pe. the typical cut off is 500 μg/l, although this varies based on the assay. however, in those over the age of 50, changing the cut-off value to the person's age multiplied by 10 μg/l (accounting for assay which has been used) is recommended as it decreases the number of falsely positive tests without missing any additional cases of pe. when a pe is being suspected, several blood tests are done in order to exclude important secondary causes of pe. this includes a full blood count, clotting status (pt, aptt, tt), and some screening tests (erythrocyte sedimentation rate, kidney function, liver enzymes, electrolytes). if one of these is abnormal, further investigations might be warranted to the issue. troponin levels are increased in between 16 and 47% with pulmonary embolism. imaging in typical people who are not known to be at high risk of pe, imaging is helpful to confirm or exclude a diagnosis of pe after simpler first-line tests are used. medical societies recommend tests such as the d-dimer to first provide supporting evidence for the need for imaging, and imaging would be done if other tests confirmed a moderate or high probability of finding evidence to support a diagnosis of pe. ct pulmonary angiography is the recommended first line diagnostic imaging test in most people. ultrasound of the legs can confirm the presence of a pe but cannot rule it out. ct pulmonary angiography ct pulmonary angiography (ctpa) is a pulmonary angiogram obtained using computed tomography (ct) with radiocontrast rather than right heart catheterization. its advantages are that it is accurate, it is non-invasive, it is more often available, and it may identifying other lung disorders in case there is no pulmonary embolism. the accuracy and non-invasive nature of ctpa also make it advantageous for people who are pregnant. on ct scan, pulmonary emboli can be classified according to the level along the arterial tree. segmental and subsegmental pulmonary emboli on both sides ct pulmonary angiography showing a "saddle embolus" at the bifurcation of the main pulmonary artery and thrombus burden in the lobar arteries on both sides. pulmonary embolism (white arrow) that has been long-standing and has caused a lung infarction (black arrow) seen as a reverse halo sign. assessing the accuracy of ct pulmonary angiography is hindered by the rapid changes in the number of rows of detectors available in multidetector ct (mdct) machines. according to a cohort study, single-slice spiral ct may help diagnose detection among people with suspected pulmonary embolism. in this study, the sensitivity was 69% and specificity was 84%. in this study which had a prevalence of detection was 32%, the positive predictive value of 67.0% and negative predictive value of 85.2%. however, this study's results may be biased due to possible incorporation bias, since the ct scan was the final diagnostic tool in people with pulmonary embolism. the authors noted that a negative single slice ct scan is insufficient to rule out pulmonary embolism on its own. a separate study with a mixture of 4 slice and 16 slice scanners reported a sensitivity of 83% and a specificity of 96%, which means that it is a good test for ruling out a pulmonary embolism if it is not seen on imaging and that it is very good at confirming a pulmonary embolism is present if it is seen. this study noted that additional testing is necessary when the clinical probability is inconsistent with the imaging results. ctpa is non-inferior to vq scanning, and identifies more emboli (without necessarily improving the outcome) compared to vq scanning. ventilation/perfusion scan ventilation-perfusion scintigraphy(a) after inhalation of 20 mci of xenon-133 gas, scintigraphic images were obtained in the posterior projection, showing uniform ventilation to lungs. (b) after intravenous injection of 4 mci of technetium-99m-labeled albumin, scintigraphic images shown here in the posterior projection. this and other views showed decreased activity in multiple regions. a ventilation/perfusion scan (or v/q scan or lung scintigraphy) shows that some areas of the lung are being ventilated but not perfused with blood (due to obstruction by a clot). this type of examination is as accurate as multislice ct, but is less used, due to the greater availability of ct technology. it is particularly useful in people who have an allergy to iodinated contrast, impaired kidney function, or are pregnant (due to its lower radiation exposure as compared to ct). the test can be performed with planar two-dimensional imaging, or single photon emission computed tomography (spect) which enables three-dimensional imaging. hybrid devices combining spect and ct (spect/ct) further enable anatomic characterization of any abnormality. low probability diagnostic tests/non-diagnostic tests tests that are frequently done that are not sensitive for pe, but can be diagnostic. chest x-rays are often done on people with shortness of breath to help rule-out other causes, such as congestive heart failure and rib fracture. chest x-rays in pe are rarely normal, but usually lack signs that suggest the diagnosis of pe (for example, westermark sign, hampton's hump). ultrasonography of the legs, also known as leg doppler, in search of deep venous thrombosis (dvt). the presence of dvt, as shown on ultrasonography of the legs, is in itself enough to warrant anticoagulation, without requiring the v/q or spiral ct scans (because of the strong association between dvt and pe). this may be a valid approach in pregnancy, in which the other modalities would increase the risk of birth defects in the unborn child. however, a negative scan does not rule out pe, and low-radiation dose scanning may be required if the mother is deemed at high risk of having a pulmonary embolism. the main use of ultrasonography of the legs is therefore in those with clinical symptoms suggestive of deep vein thrombosis. fluoroscopic pulmonary angiography selective pulmonary angiogram revealing clot (labeled a) causing a central obstruction in the left main pulmonary artery. ecg tracing shown at the bottom. historically, the gold standard for diagnosis was pulmonary angiography by fluoroscopy, but this has fallen into disuse with the increased availability of non-invasive techniques that offer similar diagnostic accuracy. electrocardiogram electrocardiogram of a person with pulmonary embolism, showing sinus tachycardia of approximately 100 beats per minute, large s wave in lead i, moderate q wave in lead iii, inverted t wave in lead iii, and inverted t waves in leads v1 and v3. the primary use of the ecg is to rule out other causes of chest pain. an electrocardiogram (ecg) is routinely done on people with chest pain to quickly diagnose myocardial infarctions (heart attacks), an important differential diagnosis in an individual with chest pain. while certain ecg changes may occur with pe, none are specific enough to confirm or sensitive enough to rule out the diagnosis. an ecg may show signs of right heart strain or acute cor pulmonale in cases of large pes – the classic signs are a large s wave in lead i, a large q wave in lead iii, and an inverted t wave in lead iii (s1q3t3), which occurs in 12–50% of people with the diagnosis, yet also occurs in 12% without the diagnosis. this is occasionally present (occurring in up to 20% of people), but may also occur in other acute lung conditions, and, therefore, has limited diagnostic value. the most commonly seen signs in the ecg are sinus tachycardia, right axis deviation, and right bundle branch block. sinus tachycardia, however, is still only found in 8–69% of people with pe. ecg findings associated with pulmonary emboli may suggest worse prognosis since the six findings identified with rv strain on ecg (heart rate > 100 beats per minute, s1q3t3, inverted t waves in leads v1-v4, st elevation in avr, complete right bundle branch block, and atrial fibrillation) are associated with increased risk of circulatory shock and death. cases with inverted t in leads v1-3 are suspected with pe or inferior myocardial infarction. pe cases show inverted t waves in leads ii and avf, but inferior myocardial infarction cases do not show inverted t waves in ii and avf. echocardiography in massive and submassive pe, dysfunction of the right side of the heart may be seen on echocardiography, an indication that the pulmonary artery is severely obstructed and the right ventricle, a low-pressure pump, is unable to match the pressure. some studies (see below) suggest that this finding may be an indication for thrombolysis. not every person with a (suspected) pulmonary embolism requires an echocardiogram, but elevations in cardiac troponins or brain natriuretic peptide may indicate heart strain and warrant an echocardiogram, and be important in prognosis. the specific appearance of the right ventricle on echocardiography is referred to as the mcconnell's sign. this is the finding of akinesia of the mid-free wall but a normal motion of the apex. this phenomenon has a 77% sensitivity and a 94% specificity for the diagnosis of acute pulmonary embolism in the setting of right ventricular dysfunction. histopathology of a pulmonary artery from autopsy. it shows a fat embolism (seen as multiple empty globular spaces on this h&e stain since its processing dissolves fat). there is a bone marrow fragment in the middle, and multiple single hematopoietic cells in the blood, being evidence of fracture as the source of the embolism. prevention further information: thrombosis prevention pulmonary embolism may be preventable in those with risk factors. people admitted to hospital may receive preventative medication, including unfractionated heparin, low molecular weight heparin (lmwh), or fondaparinux, and anti-thrombosis stockings to reduce the risk of a dvt in the leg that could dislodge and migrate to the lungs. following the completion of anticoagulation in those with prior pe, long-term aspirin is useful to prevent recurrence. treatment anticoagulant therapy is the mainstay of treatment. acutely, supportive treatments, such as oxygen or analgesia, may be required. people are often admitted to hospital in the early stages of treatment, and tend to remain under inpatient care until the inr has reached therapeutic levels (if warfarin is used). increasingly, however, low-risk cases are managed at home in a fashion already common in the treatment of dvt. evidence to support one approach versus the other is weak. anticoagulation anticoagulant therapy is the mainstay of treatment. for many years, vitamin k antagonists (warfarin or less commonly acenocoumarol or phenprocoumon) have been the cornerstone. as vitamin k antagonists do not act immediately, initial treatment is with rapidly acting injectable anticoagulants: unfractionated heparin (ufh), low molecular weight heparin (lmwh), or fondaparinux, while oral vitamin k antagonists are initiated and titrated (usually as part of inpatient hospital care)