Rules for the road: an evidence-based approach to understanding diagnostic test performance of point-of-care ultrasound for pediatric abdominal emergencies

I like to think there are many similarities between ultra-sound machines and automobiles. Just as cars come in allshapes, sizes, functions (sports-car, minivans, pickuptrucks, etc.), and price ranges, so do ultrasound machines—from the newer handheld ones now entering the market,laptop-sized ultrasound machines (which has allowedpoint-of-care ultrasound to rapidly diffuse into many areasof medicine), and to console-type ultrasound machinesused by radiology departments [1]. Learning (and teachingsomeone) how to use an ultrasound machine is not sodifferent from learning (and teaching someone) how todrive a car. Cars with automatic transmission are easier tolearn to drive than those with manual transmission; drivingin a large parking lot, or a rural area with no other cars onthe road is easier to learn than driving in places like NewYork City or London. Similarly, some ultrasound appli-cations are easier to learn than others: from fairly simple—looking at the bladder to assess urine volume prior tocollection; a little more complex—a Focused Assessmentwith Sonography in Trauma (FAST) examination; to thevery difﬁcult—systematically ﬁnding and demonstrating anormal appendix.We follow rules for the road in driving and when theyare not followed we know on a daily basis in our emer-gency departments that catastrophes can occur; ultrasoundin a clinician’s hands is no different—especially in thosejust learning to scan. Point-of-care ultrasound is a diag-nostic test like any other and understanding the diagnostictest performance characteristics of ultrasound will help usstay safe on the ‘‘road to diagnosis’’ and here is whereevidence-based medicine, or more speciﬁcally ‘‘evidence-based critical ultrasound [2]’’ can help us. Most discussionof errors in medicine have been related to preventingtreatment-related harm, with little attention given to pre-venting diagnostic errors and misdiagnosis-related harmwhich are more common than harm to patients frommedication-related errors [3]. When we look at the sensi-tivities and speciﬁcities for ultrasound to diagnose variousabdominal conditions afﬂicting children such as free ﬂuidin the abdomen in pediatric trauma [4, 5], pyloric stenosis[6–8], intussusception [9, 10], and appendicitis [11, 12], ingeneral, the speciﬁcities are higher (usually [90%) thanthe sensitivities (Table 1). How does it help us to knowthat a diagnostic test tends to have higher speciﬁcity thansensitivity?In David Sackett’s evidence-based medicine book [13],there is a handy mnemonic: A high Speciﬁcity test, aPositive result effectively ‘‘rules IN’’ disease (SpPIN). Fortests with a high Sensitivity, a Negative result effectively‘‘rules OUT’’ disease (SnNOUT). In general, ultrasoundwith its higher speciﬁcities relative to sensitivities is moreaccurate for ‘‘ruling in’’ disease, than ‘‘ruling out’’ disease(with very few exceptions). Thus, ultrasound is more reli-able in identifying pathology (e.g., free ﬂuid on a FASTexam, a shadowing appendicolith, the classic target of in-tussuception, etc.) than in completely ruling out the pres-ence of disease or abnormality (e.g., absence of free ﬂuidon FAST exam, normal muscle wall thickness of a vom-iting infant’s pylorus, etc.).However, sensitivity and speciﬁcity are traditionalmeasures of diagnostic test utility that can be difﬁcult tointerpret, particularly when the values are not very high ornot very low. Furthermore, they do not directly addressthe probability of disease in an individual patient after a