(a)  

Impression cytology (IC)
Study	Location	Year	Study design	Number of eyes	Main findings
Nolan et al. [14]	Australia	1994	Observational	Invasive squamous cell carcinoma (6) Conjunctival intraepithelial neoplasia (49) No OSSN (21)	IC can be used to demonstrate the morphology of normal and abnormal conjunctival cells. Cytology report was positive in 77% of histopathology reports in the moderate dysplasia to microinvasive carcinoma group. There were no false positives.
Tole et al. [15]	Australia	2001	Prospective case series	Squamous cell carcinoma (SCC) (1) Carcinoma in situ (2) Keratinizing dysplasia (15) Nonkeratinizing dysplasia (7)	IC is accurate in predicting the diagnosis of OSSN. Correlation rate of IC with histological findings was accurate in 80% of cases, poor in 12% of cases, and not correlated in 8% of cases. There were no false positives.
Nolan et al. [16]	Australia	2001	Retrospective observational	Intraepithelial OSSN or corneal/conjunctival intraepithelial neoplasia (142) Invasive OSSN or SCC (23)	The cytomorphology of OSSN is described in detail. For intraepithelial lesions, these include (1) keratinized dysplastic cells often accompanied by hyperkeratosis (55%), (2) syncytial-like groupings (35%), and (3) nonkeratinized dysplastic (10%) cells. Meanwhile, invasive cases had a tendency to be more highly keratinized and to have a greater degree of inflammation than the keratinized high grade intraepithelial cases but it was not possible to confidently predict invasion on IC. Sensitivity of IC in diagnosing OSSN was 78% overall but was lower (70%) when the lesion was invasive by histopathology.
Tananuvat et al. [17]	Thailand	2008	Retrospective case series	OSSN (50) including SCC (20), dysplasia (20), squamous papilloma (4), and nondysplastic changes of the epithelia (6) Pigmented lesions (5) including nevus (4) and malignant melanoma (1)	Compared with histologic findings, IC had a high positive predictive value (PPV) of 97.4% and a fair negative predictive value (NPV) of 52.9%, making it a good screening tool but inadequate gold standard. Moreover, IC is less sensitive for keratotic lesions and invasive disease.
de Nadai Barros et al. [18]	Brazil	2009	Transverse, observational	Pterygium (1) Actinic keratosis (AK) (9) Corneal/conjunctival intraepithelial neoplasia (CCIN) (9) SCC (20)	Cytological features related to malignancy were applied to determine an index score that best differentiates invasive SCC from preinvasive lesions. With an index score of ≥4.25, sensitivity was 95%, specificity was 93%, PPV was 95%, and NPV was 93% for predicting SCC. However, in two preinvasive lesions (one AK lesion and one CCIN lesion), IC sampling was not sufficiently deep to reach atypical cells and presents a limitation to diagnose disease affecting deeper tissue.
de Nadai Barros et al. [19]	Brazil	2014	Transverse, prospective, observational	Pterygia without atypical cells (19) Pterygia with associated OSSN (13)	IC showed high agreement with histopathology in detecting unsuspected OSSN in pterygia patients (sensitivity 92%, specificity 94%, PPV 92%, and NPV 94%).