Table 2.

Validity of ICD-10 codes for hyponatraemia recorded in the DNRP, using serum sodium measurements in the LABKA research database as the gold standard

Hyponatraemic serum sodium value recorded in the LABKA research database (mmol/L)			ICD-10 code for hyponatraemia recorded in the DNRP*			Validity measures, % (95% CI)
			Yes	No	Total
Overall
Na<135*	Yes		5410	301 008	306 418	Sensitivity	8 (1.7 to 1.8)
	No		440	1 879 784	1 880 224	Specificity	100 (100 to 100)
	Total		5850	2 180 792	2 186 642	PPV	92.5 (91.8 to 93.1)
						NPV	86.2 (86.2 to 86.2)
Cut-off points for increasing severity of hyponatraemia
Na<130†	Yes		4528	80 605	85 133	Sensitivity	5.3 (5.2 to 5.5)
	No		1322	2 100 187	2 101 509	Specificity	99.9 (99.9 to 99.9)
	Total		5850	2 180 792	2 186 642	PPV	77.4 (76.3 to 78.5)
						NPV	96.3 (96.3 to 96.3)
Na<125‡	Yes		3261	21 544	24 805	Sensitivity	13.1 (12.7 to 13.6)
	No		2589	2 159 248	2 161 837	Specificity	99.9 (99.9 to 99.9)
	Total		5850	2 180 792	2 186 642	PPV	55.7 (54.5 to 57.0)
						NPV	99.0 (99.0 to 99.0)
Na<120§	Yes		2061	6219	8280	Sensitivity	24.9 (24.0 to 25.9)
	No		3789	2 174 573	2 178 362	Specificity	99.8 (99.8 to 99.8)
	Total		5850	2 180 792	2 186 642	PPV	35.2 (34.0 to 36.5)
						NPV	99.7 (99.7 to 99.7)
Na<115¶	Yes		1107	2127	3234	Sensitivity	34.3 (32.6 to 35.9)
	No		4743	2 178 665	2 183 408	Specificity	99.8 (99.8 to 99.8)
	Total		5850	2 180 792	2 186 642	PPV	18.9 (17.9 to 20.0)
						NPV	99.9 (99.9 to 99.9)

*Corresponding to <133 mmol/L for infants ≤30 days of age.

†Corresponding to <128 mmol/L for infants ≤30 days of age.

‡Corresponding to <123 mmol/L for infants ≤30 days of age.

§Corresponding to <118 mmol/L for infants ≤30 days.

¶Corresponding to <113 mmol/L for infants ≤30 days of age.

DNRP, Danish National Registry of Patients; ICD-10, International Classification of Diseases, 10th revision; NPV, negative predictive value; PPV, positive predictive value.