Eligibility to donate blood

My weight is below 45kg and I was told that the minimum weight requirement to donate blood is 45kg. Why is that so?
Blood makes up about 7% of our body weight. We collect 350ml from those who weigh 45-50kg and 450ml from anyone weighing above 50kg.

Collecting less than 300ml of blood is insufficient and taking more than that from someone weighing less than 45kg may cause them to feel giddy.
I read that my haemoglobin level needs to be a minimum of 12.5g/dl for females and 13.0g/dl for males in order to donate blood. What does this mean and how do I test for it?
Haemoglobin (Hb) is a specialised protein in red blood cells that carries oxygen to the tissues and returns carbon dioxide from tissues to the lungs. Iron is essential for the manufacture of haemoglobin.

Prior to each blood donation, you will be tested for your Hb level usually through a simple finger-prick test at the blood donation site. Donors must have a Hb level of at least 12.5g/dl for females and 13.0g/dl for males before they can donate blood. This is to ensure the safety of the donor as frequent blood donations without sufficient replacement of the iron can lead to iron deficiency anaemia.
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This microsite is managed by the Singapore Red Cross, the national blood donor recruiter.
Please visit redcross.sg for more information.