A Hampton hump in a person with a right lower lobe<unk>  embolism To diagnose a<unk>  embolism, a review of clinical criteria to determine the need for testing is recommended. In those who have low risk, age less than 50, heart rate less than 100 beats per minute, oxygen level more than 94% on room air, and no leg swelling, coughing up of blood, surgery or trauma in the last four weeks, previous blood clots, or estrogen use, further testing is not typically needed. In situations with more high risk individuals, further testing is needed. A CT<unk>  angiogram (CTPA) is the preferred method for diagnosis of a<unk>  embolism due to its easy administration and accuracy. Although a CTPA is preferred, there are also other tests that can be done. For example, a proximal lower limb compression ultrasound (CUS) can be used. This is a test which is primarily used as a confirmatory test, meaning it confirms a previous analysis showing the presence or suspected presence of a<unk>  embolism. According to a cross-sectional study, CUS tests have a sensitivity of 41% and specificity of 96%. If there are concerns this is followed by testing to determine a likelihood of being able to confirm a diagnosis by imaging, followed by imaging if other tests have shown that there is a likelihood of a PE diagnosis. The diagnosis of PE is based primarily on validated clinical criteria combined with selective testing because the typical clinical presentation (shortness of breath, chest pain) cannot be definitively differentiated from other causes of chest pain and shortness of breath. The decision to perform medical imaging is based on clinical reasoning, that is, the medical history, symptoms, and findings on physical examination, followed by an assessment of clinical probability. Probability testing The most commonly used method to predict clinical probability, the Wells score, is a clinical prediction rule, whose use is complicated by multiple versions being available. In 1995, Philip Steven Wells, initially developed a prediction rule (based on a literature search) to predict the likelihood of PE, based on clinical criteria. The prediction rule was revised in 1998 This prediction rule was further revised when simplified during a validation by Wells et al. in 2000. In the 2000 publication, Wells proposed two different scoring systems using cutoffs of 2 or 4 with the same prediction rule. In 2001, Wells published results using the more conservative cutoff of 2 to create three categories. An additional version, the "modified extended version", using the more recent cutoff of 2 but including findings from Wells's initial studies were proposed. Most recently, a further study reverted to Wells's earlier use of a cutoff of 4 points to create only two categories. There are additional prediction rules for PE, such as the Geneva rule. More importantly, the use of any rule is associated with reduction in recurrent thromboembolism. The Wells score: * clinically suspected DVT – 3.0 points * alternative diagnosis is less likely than PE – 3.0 points * tachycardia (heart rate > 100) – 1.5 points * immobilization (≥ 3d)/surgery in previous four weeks – 1.5 points * history of DVT or PE – 1.5 points * hemoptysis – 1.0 points * malignancy (with treatment within six months) or palliative – 1.0 points Traditional interpretation * Score >6.0 – High (probability 59% based on pooled data) * Score 2.0 to 6.0 – Moderate (probability 29% based on pooled data) * Score 4 – PE likely. Consider diagnostic imaging. * Score 4 or less – PE unlikely. Consider D-dimer to rule out PE. Recommendations for a diagnostic algorithm were published by the PIOPED investigators; however, these recommendations do not reflect research using 64 slice MDCT. These investigators recommended: * Low clinical probability. If negative D-dimer, PE is excluded. If positive D-dimer, obtain MDCT and base treatment on results. * Moderate clinical probability. If negative D-dimer, PE is excluded. However, the authors were not concerned that a negative MDCT with negative D-dimer in this setting has a 5% probability of being false. Presumably, the 5% error rate will fall as 64 slice MDCT 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.<unk>  embolism rule-out criteria The<unk>  embolism rule-out criteria (PERC) helps assess people in whom<unk>  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 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<unk>  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<unk>  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<unk>  angiography CT<unk>  angiography (CTPA) is a<unk>  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<unk>  embolism. The accuracy and non-invasive nature of CTPA also make it advantageous for people who are pregnant. File:Computed tomograph of<unk>  vessels.On CT scan,<unk>  emboli can be classified according to the level along the arterial tree. File:SegandSubsegPE.Segmental and subsegmental<unk>  emboli on both sides File:Pulmonary embolism CTPA.CT<unk>  angiography showing a "saddle embolus" at the bifurcation of the main<unk>  artery and thrombus burden in the lobar arteries on both sides. File:CT of lung infarction with reverse halo sign, annotated.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<unk>  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<unk>  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<unk>  embolism. The authors noted that a negative single slice CT scan is insufficient to rule out<unk>  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
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