Aim: The ALAD enzyme activity ratio, which is highly sensitive to and specific for lead inhibition, can be used to diagnose lead-exposed individuals, but a reference value for it has not yet been established. Therefore, we aimed to establish a reference value for the ALAD activity ratio by improving each analytical step of the colorimetric method described by Mitchell et al. Materials and methods: Activators, pH, hemolyzing agents, and Ehrlich’s reagent were evaluated to obtain the optimal conditions for the restoration of lead-inhibited ALAD enzyme activity. Results: ZnCl2 and DTT were the preferred activators, the pH of sodium phosphate buffer was adjusted to 6.2, and distilled water was used as a hemolyzing agent. CV for the imprecision of non-activated ALAD, activated ALAD, and the activity ratio was decreased from 5.1%, 3.7%, and 7.1% to 2.6%, 1.9%, and 1.9%, respectively. Linearity improved from 20-fold dilution to 80-fold. Higher activated absorbances were observed at moderately high lead concentrations (12.5-50 µg dL−1). Conclusion: The reference value for the ALAD activity ratio based on 137 healthy men was 0.32-0.64. We think that the activity ratio could be a useful marker for screening lead-exposed individuals. It is also suggested that lead could behave as both an active and non-active metal.

Aim: The ALAD enzyme activity ratio, which is highly sensitive to and specific for lead inhibition, can be used to diagnose lead-exposed individuals, but a reference value for it has not yet been established. Therefore, we aimed to establish a reference value for the ALAD activity ratio by improving each analytical step of the colorimetric method described by Mitchell et al. Materials and methods: Activators, pH, hemolyzing agents, and Ehrlich’s reagent were evaluated to obtain the optimal conditions for the restoration of lead-inhibited ALAD enzyme activity. Results: ZnCl2 and DTT were the preferred activators, the pH of sodium phosphate buffer was adjusted to 6.2, and distilled water was used as a hemolyzing agent. CV for the imprecision of non-activated ALAD, activated ALAD, and the activity ratio was decreased from 5.1%, 3.7%, and 7.1% to 2.6%, 1.9%, and 1.9%, respectively. Linearity improved from 20-fold dilution to 80-fold. Higher activated absorbances were observed at moderately high lead concentrations (12.5-50 µg dL−1). Conclusion: The reference value for the ALAD activity ratio based on 137 healthy men was 0.32-0.64. We think that the activity ratio could be a useful marker for screening lead-exposed individuals. It is also suggested that lead could behave as both an active and non-active metal.