Table 4

Serum sodium concentration (mmol/l) describing those patients where the ICD-10 coding algorithms did and did not indicate hyponatraemia (code positive and code negative)

		Emergency department			Hospital admission
ICD-10 E87.1 coding algorithms		N	Median	IQR	N	Median	IQR
All diagnosis	+	582	123	119–126	1171	125	120–130
	−	63999	138	136–140	63328	137	135–139
Main diagnosis	+	320	122	117–126
	−	64261	138	136–140
Admission diagnosis	+				863	124	119–128
	−				63636	137	135–139
Most responsible diagnosis	+				255	120	116–123
	−				64244	137	135–139
Admission diagnosis+most responsible diagnosis	+				252	120	116–123
	−				64247	137	135–139

Code-positive and code-negative patients were significantly different (p values <0.0001) (means presented in box plot; figure 1).
ICD-10, International Classification of Diseases, 10th revision; N, number; +, code positive; −, code negative.