Objective To assess the diagnostic accuracy of lateral neck radiographs (LNR) for acute supraglottitis in adults.

Design A single centre prospective observational study.

Setting Emergency department at Kurashiki Central Hospital, Japan.

Participants Adult patients who underwent LNR to detect supraglottitis.

Main outcome measures Presence of supraglottitis, based on nasopharyngeal laryngoscopy or a follow-up telephone call, 7–30 days after the visit.

Results 140 patients had LNR during the study period. 35 patients were excluded from further analysis because of lack of consent. Of the 105 eligible patients, 21 patients (20%) were given the diagnosis of supraglottitis: 17 of 29 with a radiographic abnormality, and 4 of 76 patients without a radiographic abnormality. Three of the four cases where LNR was negative was grade 1, and all cases of grade 3 or higher had abnormal LNR. Sensitivity and specificity (95% CI) of LNR for supraglottitis were 81.0% (64.2 to 97.7) and 85.7% (78.2 to 93.2), respectively. The positive predictive value of LNR was 58.6% (40.7 to 76.5) and the negative predictive value was 94.7% (89.7 to 99.8). The positive likelihood ratio of LNR was 5.67 (3.27 to 9.82) and the negative likelihood ratio was 0.22 (0.10 to 0.51).

Conclusions LNR showed only moderate sensitivity and specificity for supraglottitis and would miss some cases of supraglottitis if the pre-test probability is high. LNR was very sensitive for grade 3 or higher supraglottitis, but would miss milder cases.

Trial registration UMIN000011928.