When verifying a hearing aid fitting, a Real Ear Measurement (REM) is used to show how well the hearing aids are matching a prescribed target, and to adjust the performance where necessary. This process is referred to as verification of hearing aids.
Most commonly in late childhood through to adult populations, one of several types of real ear measures are used for verification; Real Ear Aided Gain, Real Ear Aided Responses or Real Ear Insertion Gain (REAG, REAR or REIG). These techniques require the patient to remain relatively still for some minutes whilst the measurements at several different signal levels (and any adjustments to the hearing aid output) are being carried out. So, your question as to whether one should use REAG or RECD might be more generalisable; when should we verify using procedures that all take place via a probe-mic in the real ear (REAG,REAR,REIG), and when should we adopt a different approach?
A Real Ear to Coupler Difference (RECD) measure allows the clinician to derive the real-ear performance of a hearing aid while performing most of the verification procedures in a coupler, within a Hearing Instrument Test box (HIT). This can be ideal for use with patient populations who cannot or will not maintain stillness for the longer probe-mic procedures mentioned above, or indeed be inclined to pull at cables needed for communicating with the hearing aid and probe-mic instruments on the real-ear. The RECD does involve a single real-ear measurement (whereby a signal such as a pure-tone sweep is presented to the occluded ear of the patient, often via their ear-mould, allowing the level in the real-ear to be measured via the probe tube microphone). However, the remainder of the verification procedure is carried out in the HIT box. The next step is to measure the level of the same signal (pure tone sweep) in the coupler within the HIT box. The difference between the level in the real-ear and the coupler is the RECD. This transform can now be applied to any further measurements made in the coupler (i.e. the performance of the hearing aid that the patient is due to wear), and it allows the audiologist to derive the performance of the hearing aid in real-ear while verifying it in the coupler; the patient isn’t required to sit still and cooperate for any longer than a few moments to obtain the initial real-ear measure.
Generally this procedure relates to infants and children who may not cooperate for REAG, REAR or REIG routinely used in adults. However, it is also useful in those of any who may have mental or physical disabilities that mean sitting still for several minutes is not practical. We might also consider clinicians attending patients in their home environment or other remote consultations where travelling with REM equipment may be difficult; in this case a previously stored RECD can be used to verify the hearing aid in advance.
A second application of the RECD (but not REAG, REAR or REIG) is prior to the verification of the hearing aid.
Immediately prior to the verification of the hearing aid (but immediately after the audiogram has been measured) there is a need to convert hearing thresholds (as measured in dB HL) into dB SPL in the real ear. This step, performed by the hearing aid fitting software, is necessary so that the prescription of hearing aid gain for the real ear can be formulated (e.g. using validated formulas from NAL or DSL or other sources). When hearing thresholds are measured using insert phones, the individuals RECD (be that adult or infant patients) can be used to accurately derive the patient’s hearing thresholds in dB SPL and thus enable the hearing aid fitting software to formulate an individualised prescription of hearing aid gain. If the individuals RECD is not measured at this point, the fitting software can still prescribe hearing aid gain using the same formula, but it will not be individualised; instead, an average RECD would be used and so potential inaccuracies would ensue.