*3.1.1 Biological indicators*

Biological indicators consist of clinical, functional and histological components. The clinical indicators are xerophthalmia where it consists of two words, 'xeros' (dry) and 'ophthalmia' (eye), which refers to specific eye diseases caused by VAD [28]. It is classified into several groups with night blindness being the earliest ocular sign of VAD. The xerophthalmia classifications and its associated criteria of public health problems by WHO are highlighted in **Table 2**.


**Table 2.** 

*Xerophthalmia classifications and its associated criteria of public health problem as per WHO (adapted from [57]).* 

#### *3.1.1.1 Functional indicators*

 Night blindness (XN) or poor adaptation to the dark is a functional indicator of VAD, which is assessed by taking a history from mothers and both pregnant and lactating women. The cut-off point to indicate the deficiency in mothers and children (age 24–71 months) is ≥1% report history of night blindness [29]. Night blindness occurs if vitamin A is seriously depleted since it is responsible for vision under very low illumination [30].
