The cover test is the simplest and most reliable test for the diagnosis of strabismus.

The cover test is the simplest and most reliable test for the diagnosis of strabismus. By the time the child has his attention steadily on a target-game or on the TV, the examiner covers by hand or with a special mask for 2-3 seconds one and then the other eye, carefully observing whether there is movement of the eye which remains uncovered or if the eye just revealed is moving.

Coverage of one eye with accompanying eye movement that remains unsecured means manifest strabismus (tropia).

Coverage of one eye without associated movement of the bare eye but with eye movement from the eye that was covered, once revealed, means latent strabismus (phoria). To a child who does not have strabismus, the coverage test shall not cause any movement, nor to the eye which is uncovered, nor to the one that was covered.
The coverage test is quite sensitive and can reveal the tiniest deviations, since the movement of the eye that squints is most easily recognized when both eyes are open, by the derogation itself. Regardless of the age of the patient with strabismus, he will use the best eye to see, leaving the weaker to deviate. This occurs even when the difference of vision between the two eyes is small.

Variations of the coverage test
The test coverage can easily diagnose a manifest strabismus (tropia) but not a latent (phoria), because, as the patient concentrates on the
object or screen that he was asked, his eyes are still aligned on account of identification.

The test of alternate coverage (cross cover test or Alterna cover test) is a variation of the coverage test, in which the examiner carries the hand or the hood directly from one eye to the other, thereby preventing both eyes to
simultaneously see. Thereby, the identification is interrupted and the covered eye is directed towards the latent derogation.
Obviously we cannot see the movement if the eye is covered, but with its revelation we can observe the return of the eye in a straight line towards the target we asked the patient.

The coverage test with the use of prisms is another variant of the assay, in which prisms with increasing prismatic power are placed in front of the eye that squints until any movement in the straight ahead position that is the target fixation
is eliminated. The prisms move the image of the object towards the side the eye is facing, and when the correct force is found, the eye sees the object without having to move.