Blood supplies have dropped to critically low levels forcing the NHS Blood and Transplant service to declare its first-ever amber alert.

But why are blood stocks depleted, what does it mean for health services and can people help?

What's happening?

Blood stocks are at extremely low levels meaning that health staff have been told they may have to prioritise patients for blood to ensure there is enough for those who need it most.

The NHS likes to hold more than six days of blood in stock however overall current levels of all blood types stand at just 3.1 days while levels of O type blood, which are in greater demand because O negative blood can be used by everyone and particularly in emergencies, have fallen to just below two days - hence the warning.

Why are stocks low?

The current amber alert has a lot to do with staffing problems, an issue facing many NHS services as we go into the winter, which has been made worse by high levels of staff sickness and rising Covid-19 rates are contributing to that.

More staff, says the NHSBT, are needed to work at donor sessions and so this has impacted on the amount of blood teams have had the capacity to collect.

Maintaining blood stocks has been an ongoing challenge in the aftermath of the Covid pandemic - primarily because of staff shortages and team members being off sick but with less people visiting collection centres in towns and cities this too has had a gradual impact on donor numbers and blood stock levels.

Blood can also only be stored for 35 days, which means donations are continuously required and so any drop in numbers can very quickly lead to a shortage.

What does it mean for healthcare?

Sounding the alarm means asking hospitals to limit the use of the blood they have where they can, to protect vital stocks until supplies can be recovered.

O negative blood, where stocks are at their lowest, is the universal blood type making it vitally important during emergencies when the patient's blood type isn't known. It is also the blood type carried by emergency response vehicles, including the air ambulance, because it can be given to everyone they treat.

In terms of health care - the NHS is not saying that there won't be enough blood for emergencies, for people under going cancer treatment, who are waiting for a transplant or where it is needed in unavoidable scenarios such as a patient giving birth who has lost a lot of her own blood.

But to avoid drawing on stocks further, non-urgent operations could be cancelled until supplies have recovered, to avoid placing additional and unnecessary pressure on the limited amounts of blood available.

How long might the problem last?

The amber alert is expected to remain in place for around four weeks to enable blood stocks to be rebuilt. Levels will be regularly reassessed but the NHSBT expects this to be long enough for the situation to improve.

Wendy Clark, interim chief executive of NHSBT, said: “Asking hospitals to limit their use of blood is not a step we take lightly. This is a vital measure to protect patients who need blood the most.

"Patients are our focus. I sincerely apologise to those patients who may see their surgery postponed because of this.

"With the support of hospitals and the measures we are taking to scale up collection capacity, we hope to be able to build stocks back to a more sustainable footing."

What is being done to fix it?

Moving more staff over to help with blood donation, to make more appointments available so that more blood can be collected is among the action now being taken to alleviate the problem.

Alongside this, the service is trying to fill any vacant posts by speeding up recruitment. The NHS says it will also use agency staff where it needs to, to plug staffing gaps, while ensuring it can retain its existing workers.

Hospital and donor teams are also working closely to ensure all valuable blood the service has is being directed to where it is most needed.

Professor Cheng-Hock Toh, chairman of the National Blood Transfusion Committee, said: “I know that all hospital transfusion services, up and down the country, are working flat out to ensure that blood will be available for emergencies and urgent surgeries.

"We will continue to work closely and collaboratively with NHSBT and with surgeons and anaesthetists, in particular, to minimise any inconvenience and problems to patients."

Can the public help?

O negative and O positive donors are being asked to book in at blood donor centres should they see an appointment available - particularly as staffing levels increase, which in turn should create more blood donor sessions for people to go along to.

But crucially - with staffing levels critical and teams already struggling to run aspects of the service - people from all blood groups are being urged to keep appointments they have and attend on time if they've already got a slot booked in.

Wendy Clark added: "We cannot do this without our amazing donors. If you are O positive or O negative in particular, please make an appointment to give blood as soon as you can. If you already have an appointment, please keep it."

Who can give people and how?

To donate blood a person needs to be fit and well, between the ages of 17 and 65 and weigh between 7 stone 12 lbs (50kg) and 25 stone (158kg).

Alongside this potential donors must meet other criteria before they will be allowed to give their blood.

Those who can't donate include people with some heart conditions, those who have received blood, platelets, plasma or any other blood products after January 1, 1980, who have had cancer, had an organ transplant or have had hepatitis.

There may also be some temporary reasons why a person can't give blood and may need to wait to make an appointment. This could include people who have recently got a tattoo or piercing, have visited certain countries outside the UK within a set time frame, are pregnant or recently had a baby or because they're currently undergoing tests for a medical condition such as a heart problem.