What causes an initial false positive result in blood cross-matching, particularly in patients with a history of previous transfusions or pregnancies?

Causes of Initial False Positive Results in Blood Cross-Matching

False positive cross-match results most commonly occur due to rouleaux formation, cold-reactive antibodies, fibrin clots, or technical artifacts, with rouleaux being the most frequent culprit accounting for the majority of false positive reactions.

Primary Causes of False Positive Cross-Matches

Rouleaux Formation

• Rouleaux is the leading cause of false positive immediate-spin cross-matches, accounting for approximately 65% of false positive reactions in patients without significant red blood cell antibodies 1
• This phenomenon occurs when red blood cells stack together like coins due to increased plasma proteins, creating the appearance of agglutination without true antibody-antigen interaction 1
• Rouleaux is particularly common in patients with paraproteinemias, hypergammaglobulinemia, or elevated acute phase reactants 2

Cold-Reactive Antibodies

• Cold agglutinins and other cold-reactive antibodies cause approximately 15% of false positive immediate-spin cross-matches 1
• These IgM antibodies react at room temperature or below but are typically not clinically significant at body temperature 1
• The combination of rouleaux and cold-reactive antibodies can occur simultaneously in some patients 1

Technical and Specimen-Related Issues

• Fibrin clots in the serum sample can cause false positive reactions by trapping red blood cells and mimicking agglutination 1
• Poor specimen quality, improper centrifugation, or contamination may contribute to false positive results 1

Context-Specific Considerations

In Previously Transfused or Pregnant Patients

• Anti-HLA antibodies can interfere with cross-matching, particularly in multiparous women where these antibodies are detected in up to 30% of cases 2
• These antibodies can cause false positive reactions in whole platelet assays and must be interpreted in the clinical context 2
• The presence of anti-HLA antibodies does not necessarily indicate clinically significant red cell incompatibility 2

Antiglobulin Phase Testing

• When the antiglobulin crossmatch is performed, approximately 79% of positive results may be false positives due to laboratory artifacts rather than true incompatibility 3
• False positive antiglobulin crossmatches can occur even when IgG antibodies are present but are not clinically significant 3

Clinical Significance and Risk Assessment

True vs. False Positive Differentiation

• The immediate-spin crossmatch has a specificity of 99.86%, meaning false positives occur in approximately 0.14% of tests 1
• Laboratory protocols must distinguish quickly between true-positive and false-positive reactions to avoid unnecessary delays in transfusion 1
• Confirmatory testing using saline replacement technique can resolve rouleaux, while warming the sample to 37°C can eliminate cold-reactive antibody interference 1

Rare Causes of Missed Incompatibility

• Antibodies to low-incidence antigens (such as anti-Kpa, anti-Wra) may not be detected by standard antibody screening but can cause positive crossmatches 4, 5
• These antibodies occur in approximately 1 per 250,000 immediate-spin crossmatches and can lead to acute hemolytic transfusion reactions if missed 4
• Anti-i antibodies can rarely cause acute hemolysis despite negative immediate-spin crossmatches, though this represents less than 1 in 200,000 transfused units 6

Common Pitfalls to Avoid

• Do not assume all positive crossmatches indicate true incompatibility—systematic investigation is required to differentiate false positives from clinically significant antibodies 1
• Avoid using outdated or improperly stored reagents, as freezing and thawing can disrupt certain antigens and cause false results 2
• In patients with previous transfusions or pregnancies, consider that historical high PRA sera may not reflect current immune status, potentially leading to apparently negative crossmatches in sensitized patients 2
• Ensure proper specimen handling to prevent fibrin formation, which is a preventable cause of false positive reactions 1