A new diagnostic index for bacterial conjunctivitis in primary care. A re-derivation study

Figures & data

Table 1. Patient characteristics and pathogen species.

Characteristic	Combined data (n = 386)
Medical history
Age (years) median (IQR)	40	33;55
Female (n, %)	238	62
Hay fever (n, %)	67	17
History of allergic conjunctivitis (n, %)	31	8
Actual complaints
Itching (n, %)	184	48
Burning sensation (n, %)	230	60
Foreign body sensation (n, %)	150	39
Bilateral involvement (n, %)	99	26
Glued eyes at awakening
None (n, %)	59	15
One (n, %)	240	63
Two (n, %)	84	22
Physical examination
Redness
Peripheral only (n, %)	142	37
Whole conjunctiva ± pericorneal (n, %)	243	63
Secretion
None or clear (n, %)	302	79
Mucous or purulent (n, %)	82	21
Periorbital oedema (n, %)	105	28
Culture results
Total number of positive cultures (n, %)	140	36
Pathogens
Staphylococcus aureus (n, %)	41	29
Streptococcus pneumoniae (n, %)	44	31
Haemophilus influenzae (n, %)	18	13
Other^a (n, %)	39	27
Total pathogens (n, %)	142	100

IQR denotes interquartile range (25th and 75th centile, respectively).

^aOthers include: Pseudomonas spp., Moraxella catarrhalis, other Streptococcus spp.

Table 2. Index tests and their univariable odds ratios for the combined dataset (n = 386). Values are numbers (%), unless stated otherwise.

	Positive culture 140 (36)	Negative culture 246 (64)	Odds ratio (95% CI)
Medical history
Mean age in years (SD)	49 (17)	41 (15)	1.03 (1.01–1.04)
Female	92 (66)	146 (60)	1.31 (0.85–2.03)
Hay fever	23 (17)	44 (18)	0.91 (0.52–1.58)
History of allergic conjunctivitis	12 (9)	19 (8)	1.11 (0.52–2.36)
History of conjunctivitis	34 (24)	62 (25)	0.94 (0.58–1.52)
Actual complaints
Itching	63 (45)	121 (49)	0.85 (0.56–1.28)
Burning sensation	85 (61)	145 (59)	1.08 (0.70–1.64)
Foreign body sensation	57 (41)	93 (38)	1.13 (0.74–1.73)
Bilateral involvement	53 (38)	46 (19)	2.62 (1.64–4.19)
Glued eyes at awakening
None	10 (7)	51 (21)	Reference
One	81 (58)	159 (65)	2.83 (1.33–6.04)
Two	51 (36)	35 (14)	7.78 (3.39–17.87)
Physical examination
Redness
Peripheral only	42 (30)	100 (41)	Reference
Whole conjunctiva ± pericorneal	97 (70)	146 (59)	1.58 (1.02–2.46)
Secretion
None or clear	110 (79)	192 (79)	Reference
Mucous or purulent	30 (21)	52 (21)	1.00 (0.61–1.67)
Periorbital oedema	37 (27)	68 (28)	0.92 (0.57–1.47)

Figure 1. (Hosmer–Lemeshow) Calibration plots showing how the diagnostic probabilities of a positive bacterial culture predicted by the models correspond with the observed probabilities in ten groups of about equal size. (a) Model including age and the number of glued eyes at awakening derived on combined data (n = 386). (b) Model including age, the number of glued eyes at awakening and type of redness. Note how discrepancies between predicted and observed probabilities increased, particularly in the highest four deciles, compared to the left graph. The P-value for the fit has decreased from 0.756 to 0.235, indicating overall worse calibration.

Figure 2. Diagnostic probabilities of a positive bacterial culture as a function of age and the number of glued eyes at awakening in patients presenting to a general practitioner with symptoms of conjunctivitis. Diagnostic probabilities (prevalence) and 95% confidence limits of a positive bacterial culture as depending on age for patients with symptoms suggestive of acute conjunctivitis and who report to have no (a), one (b), or two (c) glued eyes at awakening. Probabilities and their 95% confidence limits were calculated after (parameterwise) shrinkage of the regression coefficients for age and the number of glued eyes at awakening to enhance application of the model to external populations.