Point-of-care ultrasound: Coming soon to primary care?

More sensitive, specific than x-rays for pulmonary diagnoses

The chest x-ray has traditionally been the imaging modality of choice to evaluate primary care pulmonary complaints. However, POCUS can be more sensitive and specific than a chest x-ray for evaluating several pulmonary diagnoses including pleural effusion, pneumonia, and pulmonary edema.

Pleural effusion can be difficult to detect with a physical exam alone. A systematic review showed that the physical exam is not sensitive for effusions <300 mL and can have even lower utility in obese patients.²⁸ While an upright lateral chest x-ray can accurately detect effusions as small as 50 mL, portable x-rays have sensitivities of only 53% to 71% for small- or moderate-sized effusions.^29,30 Ultrasound, however, has a sensitivity of 97% for small effusions.³¹

A 2016 meta-analysis showed that POCUS had a pooled sensitivity and specificity of 94% and 98%, respectively, for pleural effusions, while chest x-ray had a pooled sensitivity and specificity of 51% and 91%, respectively, when compared with computed tomography (CT) and expert sonography.³² POCUS evaluation for pleural effusion is technically simple, and at least one study showed that even novice users can achieve high diagnostic accuracy after only 3 hours of training.³³

Pneumonia is the eighth leading cause of death in the United States and the single leading cause of infectious disease death in children worldwide.^34-36 Pneumonia is a difficult diagnosis to make based on a history and physical examination alone, and the Infectious Diseases Society of America recommends diagnostic imaging to make the diagnosis.³⁷

The adult and pediatric literature clearly demonstrate that lung ultrasound is accurate at diagnosing pneumonia. In a 2015 meta-analysis of the pediatric literature, lung ultrasound had a sensitivity of 96% and a specificity of 93% and positive and negative likelihood ratios of 15.3 and 0.06, respectively.³⁸ In adults, a 2016 meta-analysis of lung ultrasound showed a pooled sensitivity and specificity of 90% and 88%, respectively, with positive and negative likelihood ratios of 6.6 and 0.08, respectively.³⁹

In 2015, a prospective study compared the accuracy of lung ultrasound and chest x-ray using CT as the gold standard.⁴⁰ Lung ultrasound had a significantly better sensitivity of 82% compared to a sensitivity of 64% for chest x-ray. Specificities were comparable at 94% for ultrasound and 90% for chest x-ray.⁴⁰

At least one study found novice sonographers to be accurate with lung POCUS for the diagnosis of pneumonia after only two 90-minute training sessions.⁴¹ Moreover, ultrasound has a more favorable safety profile, greater portability, and lower cost compared with chest x-ray and CT.

Pulmonary edema. Lung ultrasound can identify interstitial pulmonary edema via artifacts called B lines, which are produced by the reverberation of sound waves from the pleura due to the widening of the fluid-filled interlobular septa. These are distinctly different from the A-line pattern of repeating horizontal lines that is seen with normal lungs, making lung ultrasound more accurate than chest x-ray for identification of pulmonary edema.^42,43 When final diagnosis via blinded chart review is used as the reference standard, bilateral B lines on a lung ultrasound image have a sensitivity of 86% to 100% and a specificity of 92% to 98% for the diagnosis of pulmonary edema compared to chest x-ray’s sensitivity of 56.9% and specificity of 89.2%.⁴⁴ There is also a linear correlation between the number of B lines present and the extent of pulmonary edema.^42,45,46 The number of B lines decreases in real time as volume is removed in dialysis patients.⁴⁷

POCUS evaluation for B lines can be learned very quickly. Exams of novices who have performed only 5 prior exams correlate highly with those of experts who have performed more than 100 exams.⁴⁸

Simple, efficient screening method for abdominal aortic aneurysm

AAAs are present in up to 7% of men over the age of 50.⁴⁹ The mortality rate of a ruptured AAA is as high as 80% to 95%.⁵⁰ There is, however, a long prodromal period when interventions can make a significant difference, which is why accurate screening is so important.

AAA screening with ultrasound has been shown to decrease mortality.⁵¹ The current recommendation of the US Preventive Services Task Force (USPSTF) is a one-time AAA screening for all men ages 65 to 75 years who have ever smoked (Grade B).⁵² Despite the recommendations of the USPSTF, screening rates are low. One study found that only 9% of eligible patients in primary care practices received appropriate screening.⁵¹

Ultrasound performed by specialists is known to be an excellent screening test for AAA with a sensitivity of 98.9% and a specificity of 99.9%.⁵³ POCUS use by emergency medicine physicians for the evaluation of symptomatic AAA is well established in the literature. A meta-analysis including 7 studies and 655 patients showed a pooled sensitivity of 99% and a specificity of 98%.⁵⁴ Multiple studies also support primary care physicians performing POCUS AAA screening in the clinic setting.

For example, a 2012 prospective, observational study performed in Canada compared office-based ultrasound screening exams performed by a rural FP to scans performed in the hospital on the same patients.⁵⁵ The physician completed 50 training examinations. The average discrepancy in aorta diameters between the 2 was only 2 mm, which is clinically insignificant, and the office-based scans had a sensitivity and specificity of 100%.

Similarly, a second FP study performed in Barcelona, showed that an FP who performed POCUS AAA screening had 100% concordance with a radiologist.⁵⁶ Additionally, POCUS screening for AAA was not time consuming; it was performed in under 4 minutes per patient.^55,57