Ultrasonography is a highly operator-dependent mode of investigation, so it is very difficult to design studies to standardize its’ findings. Including USS reports from 13 independent radiologists in this study have helped to minimize subjectivity. However, having to include TAUS reports of different radiologists is also considered as a limitation of this study as this increase the variability in the skills and experience of them. All consultant radiologists were certified radiologists in the board of study for radiology in Postgraduate Institute of Medicine, University of Colombo, Sri Lanka. Since outcome prediction is more relevant at the beginning of a clinical encounter, this study considered the first USS when more than one report was available. This also maximized homogeneity because only few patients had more than one TAUS. Excluding patients who had previously undergone biliary interventions confine the results to patients with virgin biliary trees.

Knowledge of the strengths and limitations of TAUS in choledocholithiasis is important in patient management. The reported accuracy of TAUS in detecting CBDS in suspected choledocholithiasis has varied in different studies.^[9-11] From the results of the present study, it is evident that USS is highly accurate in identifying the number and location of stones, two important features that can influence the management plan. However, the stone count in TAUS was less accurate with multiple stones despite its overall excellent accuracy. Similarly, TAUS was less accurate in identifying the location of proximal stones. The possibility of stone migration between the interval between TAUS and ERCP could explain this discrepancy. These two limitations should be kept in mind when managing patients using only TAUS. This also contradicts with the common perception of TAUS being less accurate in visualizing the distal CBD due to the obscurants by bowel gas.

IHDD, which is always a reliable marker of biliary obstruction, can be very accurately detected with TAUS. Properties of stones are known to affect the outcome of endoscopic clearance; large, impacted, multiple, or intrahepatic stones are considered difficult to extract.^[12,13] The ability to identify patients with difficult stone characteristics in the initial TAUS enables their early transfer to centers with the appropriate expertise, and more advanced modalities like laser lithotripsy can be used to clear those stones. Moreover, the endotherapeutic team can be better prepared to manage difficult cases with expert personnel and equipment when the difficulty can be predicted with TAUS characteristics.

Only a few studies have been conducted to assess the accuracy of USS in detecting CCs. In a review, transabdominal USS had variable sensitivity (71%–97%) for CC diagnosis.^[14] The diagnosis of a CC requires demonstration of the continuity of the cyst with the biliary tree so that it can be differentiated from other intrabdominal cysts. This is difficult with TAUS alone due to its limited precision in visualizing the extrahepatic biliary tree.^[15] Moreover, the vast majority of patients with symptomatic choledocholithiasis have a dilated CBD. TAUS thus has very limited capacity in diagnosing CCs, necessitating more advanced imaging modalities.

According to the present study, a maximum CBD diameter of 14.5 mm can be used to predict, with a sensitivity of 90.5% and a specificity of 87.4%, the presence of a CC. Because the CBD diameter is an objective measurement, operator dependency is eliminated to a significant extent. The distinction between a dilated CBD due to distal obstruction by CBDS and a CC complicated with stone formation can be sorted out early by applying this finding, which essentially triggers a change in the management plan. The CBD diameter given in the USS can thus be used to reliably predict CC, although TAUS is inferior to other imaging modalities in sensitivity and specificity for CC diagnosis. However, this finding cannot be generalized to all CCs, as the present study did not include intrahepatic and other rare types of CCs. This finding is also limited to patients with CCs complicated with choledocholithiasis.

CBD diameter is known to increase with age,^[16,17] although this age-related increase in diameter was not found in patients with choledocholithiasis in a study conducted by Karamanos et al.^[18] Therefore, these cutoff values for the prediction of CC can be safely used without a correction for the advancing age. These findings can only be applied when the maximum diameter used is the CBD diameter, not the diameter at the hilum. Because IHDD and CBD dilatation in ERCP is a subjective finding, it is difficult to standardize the results of this study.

In Sri Lanka, TAUS remains the first-line imaging modality in investigating patients with a biliary pathology. In all tertiary care hospitals and in many secondary care hospitals, a consultant radiologist is available to provide this service. Sri Lanka has three MRI machines in government hospitals to date. In Colombo south teaching hospital, in which this study was carried out, no MRI machine is available up to date to provide MRCP for patients, despite being a premier center for hepatobiliary diseases. Few private hospitals offer MRCP as the standard imaging for all indicated patients. The government hospitals at the moment in Sri Lanka cannot provide MRCP for all indicated patients. Often, management decisions are made, solely on the findings of TAUS. However, with rapidly growing economy and developing health- care system, MRCP will soon be available more readily to Sri Lankan patients. Outcomes of this study are useful in managing patients in health-care setups with similar availability of resources.