Blood groups, antibody screening and crossmatching

Begin at the bedside, not the analyser. Ask the conscious patient to state full name and date of birth and compare these with the identity band and request; never identify by bed or another person's confirmation alone. Label the tube immediately in the patient's presence using the locally approved handwritten or electronic process. The person taking the blood completes the label. Do not pre-label tubes, transfer labels between samples or ask another staff member to label later. If identity is unknown, use one unique emergency identifier consistently across the clinical and laboratory record.

ABO is the highest-consequence blood-group system. Forward grouping detects A and B antigens on patient red cells; reverse grouping detects naturally occurring anti-A or anti-B in plasma. Group A has A antigen and anti-B, group B has B antigen and anti-A, group AB has both antigens and neither antibody, and group O has neither antigen and both antibodies. Infants, immunodeficiency, recent transfusion, transplantation, weak subgroups or cold-reactive antibodies can produce discrepancies. The laboratory resolves discordance rather than averaging results.

RhD is highly immunogenic. A D-negative recipient exposed to D-positive red cells can form IgG anti-D, creating risk during later transfusion and pregnancy. Use D-negative red cells for D-negative children and people with pregnancy potential whenever possible and follow the local emergency conservation algorithm for other recipients. Kell is also important in pregnancy; K-negative red cells are routinely selected for females with pregnancy potential unless urgency prevents this or the patient is already K positive. Other antigen matching depends on diagnosis, antibody and transfusion burden.

The antibody screen uses reagent red cells expressing clinically important antigens to detect unexpected antibodies by indirect antiglobulin testing. A positive screen triggers identification with an extended cell panel, assessment of clinical significance and selection of donor units lacking the relevant antigen. A historical antibody remains important even when it becomes undetectable because an anamnestic response can cause delayed haemolysis. Provide previous transfusion, pregnancy, transplant, antibody and reaction history; fragmented records are a patient-safety hazard.

Group and screen answers whether routine compatible blood can probably be provided, but it does not necessarily allocate a unit. Crossmatching links donor unit to recipient. Electronic issue uses validated algorithms to confirm compatibility without physically mixing samples and is restricted to patients meeting strict criteria, including no current or historical clinically significant antibody. An immediate-spin or indirect antiglobulin serological crossmatch is used according to the antibody and laboratory policy. Antigen-negative or rare units may need reference-laboratory support, so alert the laboratory early for planned surgery.

Sample age matters because transfusion or pregnancy can stimulate a new antibody after collection. Under BSH compatibility practice, a sample from someone transfused or pregnant in the preceding three months normally expires after 72 hours, calculated according to local laboratory rules. Longer validity may be accepted for selected patients without recent sensitising exposure under a validated policy. A fresh sample may be necessary after transfusion begins. Do not infer validity from a printed expiry without reporting an intervening transfusion or pregnancy.

When haemorrhage is immediately life threatening, clinical delay is more dangerous than incomplete compatibility. Take a sample before transfusion if possible, call the laboratory and state whether the patient is a child, has pregnancy potential, is pregnant or has known antibodies or special requirements. Approved emergency group O red cells are issued before testing; D status follows local policy and stock. A secure current group can permit group-specific issue before full antibody work is complete. If an antibody is known, the laboratory balances antigen-negative provision against bleeding urgency with consultant input.

Special situations require anticipation. Daratumumab can cause panreactivity in indirect antiglobulin testing for months; obtain an extended phenotype or genotype before treatment and notify the laboratory. Recent allogeneic transplant can create mixed-field grouping and donor-recipient ABO complexity. Autoantibodies can mask alloantibodies and require adsorption or reference testing. Regularly transfused haemoglobinopathy patients need extended antigen matching under disease-specific guidance. Pregnancy antibodies require quantified or titrated obstetric pathways, fetal-risk assessment and compatible maternal blood rather than a routine surgical crossmatch alone.