Using MEM values, which approach leaves the patient with a +0.50 Lag?

• A. Balance NRA and PRA
• B. RX the plus lens that yields a +0.25 Lag
• C. RX the plus lens that leaves the patient with a +0.50 Lag
• D. RX over-minus lens to create a lag

Answer: (C)

MEM values show how far the accommodative response is from the demand of a near task. A positive MEM reading indicates a lag of accommodation, and the number tells you how many diopters of lag there is. To leave the patient with a +0.50 D lag, you adjust the near lens during MEM testing and prescribe the plus lens power that makes the MEM reading +0.50 D. That lens power directly sets the accommodative demand so the measured lag matches +0.50 D.

The other approaches don’t target the MEM-determined lag: balancing NRA and PRA affects vergence and overall near workload but doesn’t establish a specific MEM lag; a plus lens that yields +0.25 D lag would give a smaller lag than desired; an over-minus lens would push toward a lag of opposite sign (a lead), not the +0.50 D lag you’re aiming for.