How Pulmonary Embolism Is Diagnosed

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Pulmonary embolism is a common medical disorder that can produce very serious consequences. Appropriate treatment, delivered expeditiously, is important for optimizing the chances of a full recovery. Giving the appropriate treatment requires making the correct diagnosis as quickly as possible.

But making the correct diagnosis of pulmonary embolus, and doing so quickly, can sometimes be a bit tricky. The most definitive tests for pulmonary embolus can be time-consuming, expensive, and entail at least some clinical risks. These tests should not be used indiscriminately.

Experts have evolved a three-step approach designed to rapidly rule out or diagnose a pulmonary embolus without exposing people to unnecessary testing. If your doctor suspects you may have had a pulmonary embolus, you can expect him or her to use this three-step diagnostic approach.

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Step One

In step one the doctor quickly assesses the likelihood that a pulmonary embolus has occurred. He or she will make this assessment by taking into account the described symptoms and the clinical circumstances in which they have occurred.

Several scoring systems have been devised for doctors to use in estimating the probability of a pulmonary embolus. The scoring system used most often is the Wells scoring system, which takes into account:

  • Whether symptoms suggesting deep vein thrombosis are present
  • Whether all other possible diagnoses seem less likely than a pulmonary embolus
  • Whether the heart rate is over 100 beats per minute
  • Whether there is a history of recent surgery or other immobilization
  • Whether there is a prior history of diagnosed deep vein thrombosis or pulmonary embolus
  • Whether there is hemoptysis (coughing up blood)
  • Whether cancer is present

Point scores are assigned to each of these seven factors and an overall Wells score is computed. With the Wells score in hand, the doctor can determine whether the probability of a pulmonary embolus is low, intermediate, or high.

PERC

If it turns out that there is only a low probability of pulmonary embolus based on this clinical assessment, the doctor may also apply an additional scoring system: the Pulmonary Embolus Rule-Out Criteria (PERC) system.

The PERC system can determine whether the probability of a pulmonary embolus is so low that further testing should be stopped altogether. It consists of eight criteria:

  • Age less than 50 years
  • Heart rate under 100
  • Blood oxygen saturation at least 95 percent
  • No hemoptysis
  • No estrogen use
  • No history of deep vein thrombosis or pulmonary embolus
  • No leg swelling
  • No surgery or trauma requiring hospitalization over past four weeks

If all eight criteria of the PERC score are present, no further testing for pulmonary embolus is recommended since the risk associated with additional testing will substantially outweigh the risk of missing a pulmonary embolus.

Step Two

If the probability of a pulmonary embolus in step one is determined to be intermediate, or if the clinical probability of pulmonary embolus is low but the PERC criteria have not been met, the next step is to obtain a D-dimer blood test.

The D-dimer test measures whether there has been an abnormal level of clotting activity in the bloodstream, such as would certainly be present if a person has had a deep vein thrombosis or pulmonary embolus.

If the clinical probability of PE is low or intermediate and the D-dimer test is negative, a pulmonary embolus generally can be ruled out and the doctor will move on to consider other potential causes for symptoms. 

A D-dimer test can only be used to rule out a pulmonary embolus, not to make the diagnosis. So if the D-dimer test is positive (or if a person’s clinical probability of a pulmonary embolus was deemed to be high in step one), it is time for step three.

Step Three

Step three consists of a diagnostic imaging study. Generally, one of three kinds of tests will be used.

CT Scan

The CT scan is a computerized x-ray technique that allows the doctor to examine the pulmonary arteries to see if there is an obstruction caused by a blood clot. A contrast agent is injected into the bloodstream during the test to help visualize the arteries.

The CT scan is accurate over 90 percent of the time in detecting a pulmonary embolus and is now considered to be the test of choice if imaging is required to make the diagnosis.

V/Q Scan

A V/Q scan (also called the ventilation/perfusion scan) is a lung scan that uses a radioactive dye, injected into a vein, to assess the flow of blood to the lung tissue. If a pulmonary artery is partially blocked by an embolus, the corresponding portion of lung tissue receives less than the normal amount of the radioactive dye.

Today the V/Q scan is usually used in people who should not be exposed to all the radiation required by a CT scan, and in those in whom the CT scan is inconclusive.

Pulmonary Angiogram

For decades the catheterization study known as the pulmonary angiogram was the gold standard for diagnosing a pulmonary embolus, but this test has now been supplanted by the CT scan.

With a pulmonary angiogram, dye is injected through a catheter placed into the pulmonary artery so that any blood clots can be visualized on x-ray. This invasive test may still be required on occasion if a CT scan or a V/Q scan cannot be used or the results from these tests are inconclusive.

In Unstable People

A pulmonary embolus may cause immediate cardiovascular collapse. In fact, a pulmonary embolus often turns out to be the culprit in younger people who die suddenly.

If a person has severe cardiovascular instability and a pulmonary embolus seems likely to be the cause, an organized three-step diagnostic plan is not feasible. In these people, treatment is often administered immediately, along with other resuscitative efforts, before a definitive diagnosis of pulmonary embolus can be made.

Differential Diagnosis

In diagnosing a pulmonary embolus, it is also important for the doctor to rule out other medical diagnoses whose symptoms can be similar to those of a pulmonary embolus. Conditions that need to be considered (that is, the differential diagnosis) often include heart attacksheart failurepericarditiscardiac tamponade, pneumonia, and pneumothorax.

The electrocardiograms, chest x-rays, and echocardiograms that are often obtained during routine clinical evaluations for suspected heart or lung disorders are usually enough to rule out these other conditions.

Even if one of these other diagnoses is made, that does not necessarily mean that a pulmonary embolus is ruled out, because a person may have two conditions at the same time—and many cardiovascular diseases increase the risk of pulmonary embolus. So if there is still reason to suspect a possible pulmonary embolus after another diagnosis is made, it is important to take the additional steps necessary to complete diagnostic testing.

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