Pulmonary embolism laboratory tests

Associate Editors-in-Chief: Ujjwal Rastogi, MBBS [mailto:urastogi@perfuse.org]

Overview
Arterial blood gas (ABG) measurements and pulse oximetry have a limited role in diagnosing PE. Also, routine laboratory testing are nonspecific. They include:
 * Leukocytosis
 * Raised erythrocyte sedimentation rate (ESR)
 * Raised serum LDH or AST (SGOT) with a normal serum bilirubin.

Blood tests
When PE is suspected, in order to exclude secondary causes of PE, a number of blood tests are done. These include: If results are abnormal, further investigations might be warranted.
 * Full blood count
 * Clotting status (prothrombin time (PT), APTT, thrombin time (TT))
 * Some screening tests (erythrocyte sedimentation rate, renal function, liver enzymes, electrolytes).

Arterial blood gas (ABG)

 * A study had shown, that in patients without prior cardiopulmonary disease, 98% of patients with PE had either an increased Alveolar-arterial oxygen difference (AaDO2) or hypocapnia.
 * In a patient without cardiopulmonary disease, having a normal AaDO2 and a normal PaCO2, the probability of PE is very unlikely (i.e. 2%).
 * Other studies have found ABG lacking enough sensitivity, specificity, positive or negative predictive value to either diagnose PE or prevent further testing in patients thought to have PE.

D-dimers
This is formed by the degradation of fibrin clot. Almost all patients with PE have some endogenous fibrinolysis, and therefore have elevated levels of D-dimer.
 * The negative predictive value (when done by ELISA) is 91% – 94%.
 * Many other diseases are associated with a mild degree of fibrinolysis, and hence an elevated D-dimer is not specific for pulmonary embolism. Disease with elevated levels of D-dimer are:
 * Pneumonia
 * Congestive heart failure (CHF)
 * Myocardial infarction (MI)
 * Malignancy

D-Dimer levels are elevated in other medical conditions such as:
 * 1) Pregnancy
 * 2) After surgery
 * 3) Hospitalized patient. Thus, most hospitalized patients should not undergo D-dimer testing if PE is suspected.

Patients who are hemodynamically stable, but have a high clinical probability or those having a high d-dimer level should undergo multidetector CT. The following table depicts the incidences of thromboembolic events in hemodynamicaly stable patients.

In low-to-moderate suspicion of PE, a normal D-dimer level (shown in a blood test) is enough to exclude the possibility of thrombotic PE. In patients with High clinical probability, the use of the d-dimer assay is of limited value.

The following flowchart summarize the role of D-dimer:

A new D-Dimer (DDMR) analyzer has shown to have higher accuracy in excluding patients with non-high clinical pre-test probability.

Brain natriuretic peptide (BNP)
In a case-control study of 2213 hemodynamically stable patients with suspected acute PE, BNP was found to have 60% sensitivity and 62% specificity.

BNP levels are typically higher in PE patients as compared to patients without PE; however, certain features limit its usefulness as a diagnostic test:
 * Many patients with PE do not have elevated BNP levels.
 * There are many alternative causes of an elevated BNP level (proving it to be nonspecific).

In hemodynamically stable patients, normal level of BNP and pro-BNP have 100% negative predictive value (NPV) for an adverse outcome. High level of BNP distinguish patients with pulmonary embolism at higher risk of complicated in-hospital duration and death, when compared with those with low BNP levels. However an isolated increase in BNP or NT-pro-BNP level, do not justify more invasive treatment regimens.

Troponin
Serum troponin I and troponin T are elevated in approximately thirty to fifty percent of the PE patients. The suspected mechanism is due to acute right heart overload. Troponin elevation is more prolonged in acute MI rather in PE and usually resolve within 40 hours after a PE event. Thus troponins are not useful for diagnosis, but there role in prognostic assessment has been proved in a meta-analysis.