Table 2.

Proportion of patients who are misclassified when laboratory testing for diabetes is repeated (World Health Organisation criteria).

Tests	True classification	Classification after repeated tests	Without bias (%)	With positive bias (%)	With negative bias (%)
Fasting Plasma Glucose Biological variation: 5.7% Analytical variation: 2.5%	Normal (<6.1 mmol/L)	Normal	96.8	87.1	99.5
		Impaired	3.2	12.6	0.5
		Diabetes	0	0.3	0
	Impaired (6.1–6.9 mmol/L)	Normal	27.2	6.3	59.7
		Impaired	63	57.6	38.8
		Diabetes	9.8	36.1	1.5
	Diabetes (≥7 mmol/L)	Normal	0.3	0	2
		Impaired	10.2	2.4	22.2
		Diabetes	89.5	97.6	75.8
OGTT Biological variation: 16.7% Analytical variation: 2.5%	Normal (<7.8 mmol/L)	Normal	91.5	85.5	95.4
		Impaired	8.5	14.4	4.6
		Diabetes	0	0.1	0
	Impaired (7.8–11.0 mmol/L)	Normal	26.9	17.4	37.8
		Impaired	61.7	62.7	56.1
		Diabetes	11.4	19.9	6.1
	Diabetes (≥11.1 mmol/L)	Normal	1	0.5	1.8
		Impaired	15.9	10.3	22.1
		Diabetes	83.1	89.2	76.1
HbA1c Biological variation: 1.8% Analytical variation: 2%	Normal (<6%)	Normal	93.2	80.6	99.1
		Pre-diabetes	6.8	19.3	0.9
		Diabetes	0	0.1	0
	Pre-diabetes (6–6.4%)	Normal	23.4	6.9	56.7
		Pre-diabetes	70.3	73.6	42.5
		Diabetes	6.3	19.6	0.8
	Diabetes (≥6.5%)	Normal	0	0	0.5
		Pre-diabetes	10.3	3	24.6
		Diabetes	89.7	97	74.9
HbA1c Biological variation: 1.8% Analytical variation: 3.5%	Normal (<6%)	Normal	89.2	77.3	97
		Pre-diabetes	10.7	22	3
		Diabetes	0.1	0.7	0
	Pre-diabetes (6–6.4%)	Normal	28.8	13.4	54.7
		Pre-diabetes	60	61.5	42.4
		Diabetes	11.2	25.1	2.9
	Diabetes (≥6.5%)	Normal	0.4	0.1	2.1
		Pre-diabetes	12.5	5.8	22.9
		Diabetes	87.1	94.1	75

A between-laboratory positive bias of +7% and +2.5% were introduced for the fasting plasma glucose/oral glucose tolerance test, and HbA1c, respectively. A between-laboratory negative bias of −6% and −3% were introduced for the fasting plasma glucose/oral glucose tolerance test, and HbA1c, respectively.