A blood service runs a supply chain for a product with a short shelf life, no substitute and a donor base that must be persuaded rather than scheduled. Everything about how it operates follows from that.
The product expires quickly
Whole blood is separated into components, and each has its own storage life. Red cells last a matter of weeks refrigerated, while platelets last only days.
Plasma can be frozen and stored far longer, which is why plasma shortages behave differently from red cell shortages.
Because the clock starts at donation, a service cannot build a large buffer in good periods to cover a bad one. Stock has to be replenished continuously.
Groups have to be matched
Blood is classified by group and by rhesus status, and a transfusion has to be compatible with the recipient.
That means a headline stock figure can conceal a shortage, since a surplus of one group does not help a patient who needs another.
Certain groups are used disproportionately in emergencies because they can be given before a patient's own group is known, so demand for them exceeds their share of the population.
Demand is partly predictable
Planned surgery is scheduled and therefore forecastable, and services work with hospitals to know what is coming.
Trauma is not. A major incident can consume in an afternoon what a region collects in a week, which is why a reserve is held at all times.
Seasonal patterns compound this. Holiday periods reduce donor attendance while road injuries rise, so the two curves move apart at the same moment.
Collection is managed, not just requested
Appeals are targeted at particular groups and particular regions rather than issued generally, because an untargeted surge produces units that expire unused.
Donors are given appointments and intervals between donations, both to protect their own iron levels and to spread collection evenly across the year.
Eligibility criteria exist to protect recipients and donors alike, and they change over time as testing improves and evidence is reviewed.
Testing and processing sit between the two
Every donation is tested for transmissible infections and confirmed for group before it can be released to a hospital.
Processing separates components, filters out white cells in many systems, and labels each unit so it can be traced to its donor and its recipient.
This takes time, which is why a donation given today does not reach a patient today, and why appeals are made before a shortage becomes acute rather than during one.