Clinicians now have two point-of-care testing devices that can measure a patient’s tear osmolarity, the Tearlab and the I-Pen (I-Med Pharma) osmometers. But clinicians should be careful to avoid using the readings interchangeably, according to new research being presented at this year’s ARVO conference in Vancouver.

This Thursday morning, researchers will present new findings that suggest the I-Pen provides significantly higher osmolarity results compared with the Tearlab device. The team from Ludwig Maximilians University in Munich, studied 51 healthy subjects—none of whom had clinically evident dry eye—using each device. Tearlab measurements, derived from the tear meniscus, came in at a mean of 294.6mOsm/l for both eyes. The I-Pen, collected from the palpebral conjunctiva, showed a mean osmolarity of 302.1mOsm/l. For both devices, the readings ranged between 268mOsm/l and 394mOsm/l.

The researchers speculate that the location of the testing—tear meniscus vs. the palpebral conjunctiva—could account for the difference.

If clinicians use Tearlab’s cut-off value of 308mOsm/l for normal osmolarity, 98% of the study participants would be considered normal, compared with only 68% when testing with the I-Pen. Thus, the researchers conclude that doctors should consider using a higher cut-off value (between 316mOsm/l and 320mOsm/l) when testing patients with the I-Pen.