Every drop of blood is precious, but they are not quite equal.
Some blood types can be hard to come by. For every 1,000 people who give blood in Singapore, only one person carries the AB negative blood type (0.1 per cent).
In general, donors with negative blood types are rare, forming only 1.5 per cent of the total pool in 2016.
To keep some of these for a rainy day, the Health Sciences Authority's (HSA) blood services group deep-freezes the red cells of rare blood types at -80 deg C.
This way, the red cells' shelf life can be extended from the usual 42 days to 10 years, said Ms Sally Lam, laboratory director of blood supply management at HSA, which runs the national blood service.
Otherwise, red cells are kept at 1 deg C to 6 deg C in fridges or cold rooms.
Platelets are kept at 20 deg C to 24 deg C in large incubators with trays that gently rock back and forth, for up to five days. Ms Lam explained that platelets need to be "agitated" constantly, or they would clump up.
Plasma is quick-frozen and refrigerated at -35 deg C for up to a year.
Only blood that gets the all-clear after laboratory tests are stored, said HSA. Blood that is unsuitable for use will be disposed of properly.
The rejection rate is low - less than 3 per cent to 4 per cent of all donated blood components. Some reasons for rejection include a positive result for an infection or that the plasma was too fatty.
Donors can also ask the HSA to retract their blood through a 24-hour call line. They do not have to give a reason for making such a request.
At hospitals, there are measures to ensure blood products are used properly and wastage is minimised.
Singapore General Hospital, for instance, has a guideline on the number of blood units required for each specific procedure, said Associate Professor Tien Sim Leng, who heads the hospital's blood bank. "To minimise wastage, blood is issued only when the patient is in the operating theatre and needs blood."
Medical institutions such as those under the SingHealth group, which includes National Heart Centre Singapore (NHCS), also have a committee overseeing blood transfusion.
Assistant Professor Tan Teing Ee, director of NHCS' cardiothoracic surgery intensive care unit, added that blood transported to the operating theatre is kept in fridges there and "will not be taken out unless it is meant to be used".
"If the blood product is out of the refrigerator for too long, it cannot be returned and thus has to be discarded," he explained.
If the blood in the refrigerator remains untouched after an operation, it will be transported back to HSA to be crossmatched to other patients.
"We try to minimise the use of blood products in surgical procedures... as blood transfusion carries risks," Prof Tan added.
Taking supplements and stopping blood-thinning medication before surgery are some ways to help to reduce patients' need for transfusions.
Prof Tan noted that expired blood, unfortunately, has to be thrown away. "This is sometimes unavoidable on days when there is more donor blood than what is required."