Lung allocation based on the Composite Allocation Score (CAS): Questions and answers for patients and caregivers
Welcome to our question-and-answer page for the new lung allocation policy that will take effect in early 2023. This policy will use an updated approach known as continuous distribution. Continuous distribution will improve the organ matching process by considering all factors about each transplant candidate in a single score.
Below are answers to the most common questions from people seeking a transplant for themselves or a loved one. If you have a more general interest in transplantation, we hope you find it helpful as well.
The team at your transplant hospital will always be your first and most complete source of information regarding your current medical condition and treatment options. If you have other questions or comments, contact the Organ Procurement and Transplantation Network (OPTN) Patient Services line at (888) 894-6361 or submit them here.
The lung allocation policy that has been in place several years will remain in effect until early 2023. The highlights of the current policy are as follows:
- Every lung transplant candidate age 12 or older receives a calculated lung allocation score (LAS). The score reflects how urgently each candidate needs a transplant and his or her chance of success following a transplant. The LAS is an important factor in deciding who gets organ offers in what order when a donor lung becomes available.
- Lung candidates under the age of 12 do not get an LAS. Instead, they are listed as either Priority 1 or Priority 2, based on how urgently they need a transplant. Those who need a transplant more quickly are listed as Priority 1. All other lung candidates in this age range are Priority 2.
- In addition to their medical urgency, transplant candidates get priority for the lung(s) being offered based on factors such as their blood type, as well as how far their transplant hospital is from the donor hospital.
- If two or more candidates have a tied score, the person who has waited the longest for a transplant will get an organ offer before someone who has waited less time.
Although the current policy has saved many lives and enhanced transplant survival over a number of years, it can be improved.
Under the current policy, each matching factor is used to form a series of rules that set the order of patients to be offered a lung transplant. This system can sometimes put too much emphasis on one factor when others may also be relevant.
For example, the current policy provides lung offers first for candidates who have the same blood type as the donor, with additional ordering based on their LAS. After that, other candidates who have a compatible, but not identical blood type match receive lung offers. This approach may lead to a situation such as illustrated here.
LAS matching of candidates based on blood type
In this example, Candidate 1, who has the same blood type as the donor, gets the first organ offer. However, Candidate 2, with a compatible blood type, would not get an earlier offer, even if s/he has a higher LAS and is listed at a transplant hospital closer to the donor hospital. In this way, the traditional approach may not ideally account for all key factors that could result in a good match.
The new lung allocation policy will use the same patient factors already in the current matching system, while adding a few new attributes such as living donor status, pediatric status, and logistics between the transplant hospital and donor hospital. These factors are combined into a Composite Allocation Score (CAS). The CAS is individual for each patient and each organ offer. CAS point values represent each of the factors used to match organ offers with transplant candidates. The people who have the highest number of points for that organ offer will have the highest priority.
How is the CAS calculated?
The CAS uses objective medical information about your needs and medical condition. It also uses objective medical facts about the potential organ donors that may be a match for you.
The score weighs the different factors used to make the match. This means each factor will get a certain number of potential points, which are then added together to make up a maximum score of 100 points.
Some factors are more important in matching and will be worth more points within the overall score, while others get fewer points. Medical experts have carefully determined the weight for each factor based on the input from the transplant community as well as detailed statistical information.
Half of the new CAS (up to 50 points) will come from attributes similar to those used in the current LAS:
- Up to 25 points will be based on each candidate’s medical urgency – how quickly he or she may need a transplant.
- In addition, up to 25 points will be based on the candidate’s likelihood of surviving at least five years if transplanted.
As many as 15 points are reserved for candidates who are hard to match for some or most organ offers. This extra priority is meant to give more opportunities for lung offers that could be a match. There are three categories, each of which make up as many as five points toward the overall CAS:
- Blood type. Candidates with harder-to-match blood types will have more points than those who easier to match with potential donors.
- Calculated panel reactive antibody (CPRA) results. CPRA is a medical test that measures how likely a person is to have an immune system rejection to most organ offers. The higher the CPRA value, the greater the risk of rejection. For example, a person with a CPRA of 80 could not be a match with organs from 80 percent of donors. Patients with a high CPRA (harder to match) will get more points to qualify for offers that may match them. People with a low CPRA (easier to match with most donors) will have fewer points, or none at all, in this category.
- Candidate’s height. Donor lungs must be a reasonable size match within the patient’s chest. Patients who are much shorter or taller than average tend to get fewer matching lung offers than those who are of more common height. For this reason, candidates who are either very short or very tall will have more points in order to have better access to donors that could be a match.
Next, up to 25 points are reserved for candidates who have special needs for access to a transplant:
- Candidates younger than 18 years old at the time of transplant listing will receive 20 points.
- Candidates who were living organ donors will receive five points.
The final portion of the CAS, worth as many as 10 points, reflects the effort needed to preserve and transport the lung(s) from the donor to the recipient hospital. Lungs are only viable for a few hours between donor and recipient. The shortest possible travel time and distance often boosts the chance of a successful transplant.
There are two parts to the transplant logistics, each of which can be as much as five points:
- Travel efficiency. This estimates the arrangements and expense needed to transport the lung(s) from the donor hospital to the transplant hospital. Donor/candidate matches with the greatest travel efficiency will be assigned the highest number of points (for example, if the donor and candidate are within the same hospital or at very nearby hospitals, or if the lung(s) can be transported by road instead of flight.)
- Proximity efficiency. This estimates factors other than travel in transporting the donor lung(s). The donor offers with the least complicated arrangements will be assigned the highest number of points (for example, matches that result in the shortest possible organ preservation time between donor and transplant hospital).
Could any person have a CAS of 100?
No, transplant candidates could never get the total maximum score for each factor. For example, no one person would be under age 18, be a prior living donor, and also have the maximum disadvantage in terms of height, blood type and CPRA level.
For some attributes, if you do not qualify, you will not get any points for that item within the total score. For example, if you are not a prior living donor, you would not get any of the points reserved for living donors. Similarly, any transplant candidate listed after age 18 will not get any points reserved for pediatric status.
For other factors, you might receive some number of points, but not always the maximum amount. For example, a person might get one point based on height disadvantage, 0.4 points based on blood type rarity and one point based on CPRA score, totaling 2.4 out of a possible 15 points.
For the parts of CAS that do not depend on the specific organ offer, many candidates will have scores around 20 to 25. In rare cases, candidates may have a CAS as high as 40 to 45 before points are added for the logistics of the match with the donor.
Because the distance from donor to transplant hospital accounts for as much as 10 points within the total CAS, you will not have the same score across all organ offers. Your score could be lower on one match run for a donor at a hospital 500 miles from your transplant program, then very high on a match for the next donor at a hospital 50 miles away.
How is CAS used in organ offers? What if I have the same score as someone else?
The OPTN’s computerized matching system will give the highest priority to the candidates with the highest total CAS for each lung offer. If the offer is not accepted for the person with the highest CAS, it will then go to the candidate with the next-highest score. The offers will be made in that order until a transplant program accepts the offer, or until, for whatever reason, the lung(s) cannot be transplanted.
As with the current lung allocation policy using the LAS, two or more candidates could have the same exact CAS. (The CAS can have values going out as far as four decimal places. So for example, you could have a score of 20.1274 or 23.3837.) If there is a tie among candidates, waiting time for transplant is used as a tiebreaker. The person who has been listed longest for a transplant would receive an organ offer before others with the same CAS but who have been listed a shorter time.
The factors that go into the LAS under the current system will make up half (a maximum of 50 points) of the total CAS. This is split evenly into medical urgency (up to 25 points) and expected post-transplant survival (up to another 25 points). Keep in mind that most candidates will not receive the maximum score totals available for either component.
A set of medical tests or observations are put into a formula (see list below). These are proven statistically to affect how soon people need a transplant and how likely they would be to survive at least five years after a transplant.
In addition, each candidate age 12 or older is included in a diagnosis group that represents their need for a transplant. There are four diagnosis groups. Each of those groups has been shown statistically to have similar patterns of medical urgency and post-transplant survival. As a result, each group gets a different amount of weighting in the CAS formula to represent how that group of diseases affects overall medical urgency and survival rates.
What do I do if my child has an adolescent exception?
In the current policy, children younger than age 12 only get a pediatric score rather than an LAS. OPTN policy does, however, allow lung transplant programs to apply for an exception status for children younger than 12 to qualify for an LAS and thus have the same access to adult donor offers that children between the ages of 12 and 18 have.
The CAS will apply to all transplant candidates, regardless of their age. Therefore, there will no longer be a need for an adolescent exception in order to ensure access to specific types of donors. Additionally, all candidates added to the list before they reach 18 years of age will receive 20 points based on their pediatric status.
What tests/observations are used to determine urgency and expected survival?
- Pulmonary artery pressure: The pressure the heart generates to pump blood through the lungs. This pressure may be high in some people with serious lung disease.
- Oxygen at rest: The amount of oxygen needed at rest to maintain enough oxygen in the blood. People with severe lung disease may need additional oxygen.
- Age: Age at the time lungs are offered.
- Body mass index: A measure of body fat based on height and weight. When combined with other medical test results, body mass index helps assess health status.
- Functional status: A way to measure the effects that lung disease has on performing routine daily tasks.
- 6-minute walk distance: How far you can walk in 6 minutes is a measure of functional status.
- Assisted ventilation: An external device connected directly to the patient to help move air into and out of the lungs. The use of a ventilator to aid breathing is a measure of disease severity.
- Serum creatinine: A measure of kidney function. High creatinine levels reflect impaired kidney function, which sometimes happens along with severe lung disease.
- Diagnosis: Different kinds of lung diseases vary in how effective a lung transplant is in treating them. Therefore, diagnosis is part of the composite allocation score.
- PCO2 and change in PCO2: The amount of carbon dioxide in the blood. When the lung isn’t able to exchange oxygen and carbon dioxide as it should, the PCO2 level may increase.
- Bilirubin: A substance made by the liver when it breaks down old red blood cells. High bilirubin is a marker for right heart failure that can occur with lung disease.
- Cardiac index: A measure of how well the heart is pumping blood. A low cardiac index means the heart is not keeping up enough blood circulation. Note: This is used in calculating the post-transplant portion of the score, but not for waitlist urgency.
Children younger than age 12 are listed either as Priority 1 or Priority 2 based on measures of their current level of illness. The criteria for each of these statuses will not change from the current policy.
The factors used to calculate medical urgency and transplant outcome for candidates age 12 and older do not apply to patients younger than 12. For this reason, their scores will be different and will be a fixed value based on their priority status.
Priority 1 candidates have a higher level of medical urgency than those who are Priority 2. Thus, Priority 1 will have a higher fixed score for medical urgency (1.9073 points) than Priority 2 (.4406 points).
The part of the score addressing post-transplant outcome will be the same for all candidates younger than age 12, regardless of priority. It will be a fixed amount of 18.6336 points.
Not always. The OPTN matching system sets the order of candidates to get lung offers. However, the offer may not be accepted for the top-ranked person or even for a number of others. Reasons for deferring an organ transplant vary, but they can include:
- the patient’s inability to be transplanted at the time of the offer;
- not having all of the transplant team available to recover and transplant the organ; and/or
- medical facts about the donor (such as age, likely organ transport time, transmissible disease risk, etc.) that may affect the success of transplanting the particular patient getting the offer.
The medical staff at your transplant program reviews detailed medical information about the donor for all lung offers identified for you. The transplant team makes the final decision to accept or turn down any offer. If the organ is not accepted for the first patient on the match, it is then offered to the next candidate. This continues until a match is found, or until the lung(s) can no longer be placed.
If your transplant team believes that your CAS does not truly reflect your degree of illness, they may ask a group of expert reviewers, known as the Lung Review Board, to review your situation.
The Lung Review Board will consider the information provided and assess how your condition compares to other candidates who have the score your team is requesting. The board then decides whether to approve or deny the request. Keep in mind that since the CAS combines a number of factors, the board may approve an adjustment in one or more parts of the score (for example, an increase in medical urgency priority) while other parts remain the same.
What is the Lung Review Board?
The Lung Review Board is a national group of transplant physicians and surgeons, established by the OPTN. The board reviews requests from transplant hospitals to grant priority in exceptional cases. These requests are created when the transplant team believes that the assigned score does not represent the severity of the case. The Lung Review Board looks at the medical facts of each case, and decides whether to approve or deny the request. Members of the board do not see names or details that identify the patient or hospital.
It may help you to learn about the new system and what it means for you. The CAS is different enough from the LAS that there is not a direct way to compare the two scores. In general, however, the priority you have under LAS should be similar to your priority under the new policy.
Your transplant program should have all the information needed to calculate most of your score before the new policy takes effect. If they need an updated test result or additional information, they will contact you. Remember that part of the CAS will be how far your transplant hospital is from the donor hospital, so it will not be the same for every available organ.
The OPTN designs and updates the lung allocation system by studying scientific data on lung transplantation. It gets expert advice on various lung diseases and seeks input from everyone interested – including transplant patients and their caregivers. The new system is expected to improve how the limited supply of donor lungs is used to help as many patients in need as possible.
How often will the transplant program update my medical information?
In general, your transplant team will update your medical information at least every six months. Your transplant program already collects most of the needed information on a regular basis.
If you are receiving some form of highly advanced treatment that usually requires you to be in the hospital, your program must update those lab values at least once every 28 days. This will ensure your score reflects the most timely information about your condition.
Your hospital may also update your information in the system any time your transplant team thinks it is necessary to reflect a change in your condition.
This system has been improved over time, and it will continue to be studied for further improvement. In organ transplantation, as in all scientific fields, new studies are taking place all the time to learn how to save more lives.
The lung allocation system is reviewed regularly to see what can be improved. In fact, the new policy is designed to allow faster and simpler improvements, such as changing the weight of one or more factors in the CAS. Your transplant team will keep you informed of changes to the system and what you may need to do.