Final answer:
To collect a 5ml tube from an 18-month-old, it is recommended to use a small diameter glass tube (capillary tube) due to the risk of blood loss.
Step-by-step explanation:
When collecting a 5ml tube from an 18-month-old, it is recommended to use a small diameter glass tube (capillary tube) due to the risk of blood loss. Capillary action, which draws blood into the tube, can be used for blood collection in this age group. This method is commonly used in medical analysis.