Visual and auditory cortical evoked potentials in interictal episodic migraine: The predictive value of a proposed diagnostic test should always be corrected for prevalence

Positive predictive value (PPV) can easily be calculated with the following formula if sensitivity, specificity and prevalence are known (1):

It is important to take disease prevalence within the target population (pre-test probability) correctly into account. Authors, referees and editors should be aware of this to prevent the publication of erroneous results.

Ambrosini et al. (2) have replied to our letter (3) regarding their recent publication (4), and they still claim that the combination of visual evoked potentials (VEP) and intensity dependent auditory evoked potentials (IDAP) have a high diagnostic value in migraine. However, recalculation based on the sensitivity and specificity reported by Ambrosini et al. (3), and the expected population-prevalence equal to 15% for migraine, yields disappointingly low PPV for VEP, IDAP and the combinations of IDAP and VEP (Table 1). Indeed, the PPV of an abnormality of at least one of VEP and IDAP is only 30.7%, far below the 94.1% value given by Ambrosini et al. A PPV of 94.1 % would require considerably higher sensitivity and specificity than those reported by Ambrosini et al.

Table 1.

Recalculation of PPV based on data from Table 3 in Ambrosini et al. (4), using a population prevalence equal to 15% for migraine (column 4). The high PPVs reported by Ambrosini et al. (column 3) were seemingly based on the high migraine prevalence within their different study groups (column 1).

Tests	Migraine prevalence (%) in the study by Ambrosini et al.	Sensitivity and specificity (%) reported by Ambrosini et al.	PPV (%) reported by Ambrosini et al.	PPV (%) given 15% migraine prevalence
VEP 50 × 5	75.0	61.0 and 77.9	89.2	32.8
VEP 100 × 6	53.8	61.4 and 77.9	76.4	32.9
IDAP	79.2	45.7 and 87.2	93.2	38.7
VEP + IDAP (positivity to at least one test)	86.5	83.4 and 66.7	94.1	30.7
VEP + IDAP (positivity to both tests)	86.5	24.6 and 90.9	94.5	32.3

PPV: Positive predictive value. VEP: Visual evoked potentials, IDAP: Intensity-dependence of auditory evoked potentials.

Ambrosini et al. also suggest that regional differences in solar radiation could be of importance in migraine (2). This hypothesis should be investigated with rigorous methods. We contend that matched controls, rigid inclusion and exclusion criteria, and blinding of both recording and analysis, are necessary to obtain reliable results. Blinded procedures are considered important in most research fields (e.g. 5, 6), and this is obviously the case also in the field of migraine pathophysiology.