Auto Control in Automated Pre-Transfusion Crossmatching
Direct Answer
The auto control (autocontrol) in automated crossmatching serves to detect autoantibodies coating the patient's own red blood cells, but has limited clinical utility in routine pretransfusion testing and can be safely omitted in most cases, as it has a low predictive value (3.6%) for detecting clinically significant alloantibodies. 1
Role of the Auto Control
The auto control is a test that detects whether the patient's own red blood cells are coated with antibodies (positive direct antiglobulin test). It functions as:
- A screening tool for autoantibodies that may complicate antibody identification when present 1
- A parallel test to the antibody screen that theoretically could detect early immune responses to recently transfused red cells 2
- A quality control measure in the crossmatch procedure 1
Interpretation of Results
Negative Auto Control
- Indicates no detectable autoantibodies coating the patient's red blood cells 1
- Proceed with standard crossmatching protocols - immediate spin crossmatch is sufficient for antibody screen-negative patients 3
- Electronic crossmatching is appropriate when the antibody screen is negative and the patient has no history of clinically significant antibodies 3
Positive Auto Control
When the auto control is positive but the antibody screen is negative:
- The predictive value for clinically significant alloantibodies is only 3.6% (25 out of 684 cases in one large study) 1
- Most positive results (66.6%) yield non-reactive eluates upon further testing 2
- Warm autoantibodies are the most common finding (detected in 192 of 778 evaluated samples), not alloantibodies 2
Clinical approach for positive auto control:
- Perform Rh subgroup phenotyping (C/c, E/e) on the patient 4
- Select crossmatched units with the lowest agglutination reaction grade - this "least incompatible" approach combined with Rh phenotype matching does not adversely affect transfusion efficiency 4
- Do not delay transfusion if clinically indicated, as the risk of missing a clinically significant antibody is minimal 1, 2
- Monitor post-transfusion hemoglobin increment - studies show no significant difference in Hb change per unit between auto control-positive and auto control-negative patients when using this approach 4
Evidence for Omitting the Auto Control
Low Clinical Yield
- In 56,090 autocontrols performed, only 25 had potential clinical significance when the antibody screen was negative 1
- Only 11 cases might have resulted in shortened RBC survival if the serologic abnormality had not been detected 1
- The antiglobulin crossmatch itself has even lower predictive value (2.8%) compared to the auto control 1
Safety Data
- A prospective study of 10,899 red cell transfusions where the antiglobulin crossmatch was omitted (when antibody screen was negative) showed no clinical or serological evidence of hemolysis, even in cases where post-transfusion testing revealed incompatibility 5
- 79.2% of "incompatible" results were false positives upon further investigation 5
Common Pitfalls and Caveats
When the Auto Control IS Clinically Important
- Delayed hemolytic transfusion reactions (DHTRs): Eight cases in one study represented DHTRs where alloantibodies caused in vivo sensitization - the auto control detected these early 1
- Patients with sickle cell disease: These patients require extended antigen matching (C/c, E/e, K, Jka/Jkb, Fya/Fyb, S/s) regardless of auto control status 6, 7
- Recent transfusion history: Patients transfused within 14 days warrant closer scrutiny of positive auto controls 2
Avoiding Errors
- Never rely solely on agglutination grade - always combine with Rh subgroup phenotyping when auto control is positive 4
- Do not confuse auto control with antibody screen - a negative antibody screen is more clinically relevant than the auto control status 1, 5
- Ensure proper patient identification using two independent identifiers before sample collection 7
Practical Algorithm
For routine pretransfusion testing:
- Perform ABO/RhD typing and antibody screen 7, 3
- If antibody screen is negative:
- If auto control is positive (when performed):
The elimination of the autocontrol from routine pretransfusion testing carries minimal risk and is more cost-effective than maintaining it as a routine test. 2