How many blood samples are required for a crossmatch (cross-matching)?

Number of Blood Samples Required for Crossmatch

For standard blood crossmatching, only one blood sample is typically required, but a second sample may be needed in specific circumstances such as recent transfusion or pregnancy within the previous 3 months, or when no valid historical sample is available for comparison 1.

Standard Requirements for Blood Crossmatching

Single Sample Sufficiency

• One properly collected blood sample is sufficient for a standard crossmatch when:
  • The patient has no history of transfusion in the past 3 months
  • The patient has not been pregnant in the past 3 months
  • There is a valid historical sample available for comparison 1

When Two Samples Are Required

• A second sample may be required in the following circumstances:
  • No valid historical sample exists for comparison
  • Electronic transmission of results with no manual intervention cannot be guaranteed
  • Patient has received blood transfusion within the previous 3 months
  • Patient has been pregnant within the previous 3 months (sample validity reduced to 72 hours) 1

Sample Collection and Processing

Proper Sample Collection

• Blood samples must be collected and labeled at the patient's side
• Minimum identifiers required include:
  • Full name (surname and forename)
  • Date of birth
  • Hospital unique identification number 1

Sample Validity

• Standard crossmatch samples are typically valid for extended periods (often months) if the patient has no history of transfusion or pregnancy
• If a patient has received a blood transfusion or been pregnant within the previous 3 months, the sample is only valid for 72 hours from collection 1

Types of Crossmatching

Serological Crossmatching

• Traditional method involving laboratory testing of donor and recipient blood
• Immediate spin crossmatch is used for antibody-negative patients
• Antiglobulin crossmatch is used when clinically significant antibodies are present 2

Electronic Crossmatching

• Introduced in 1992, this method involves no serological testing
• Relies on computer validation to ensure correct ABO/RhD type blood is issued
• Shifts the emphasis for safety from the crossmatch to the antibody screen 3
• Considered safe practice and brings benefits to both laboratory and patient 3

Safety Considerations

Preventing Errors

• The primary reason for potentially requiring two samples is to prevent wrong-blood-in-tube errors and the risk of ABO incompatibility 1
• Positive patient identification is paramount prior to any blood transfusion 4
• In a study of 65,628 samples, 1.64% were rejected for mislabeling, highlighting the importance of proper sample collection 5

Special Circumstances

• For platelet transfusions, crossmatching is generally not necessary as they contain few red cells 4
• For patients with alloimmune refractory thrombocytopenia, platelet crossmatching techniques may be needed to identify histocompatible donors 4

Common Pitfalls to Avoid

• Overlooking recent transfusions or pregnancies which significantly reduce sample validity periods
• Inadequate patient identification leading to wrong-blood-in-tube errors
• Failure to check for historical samples that could eliminate the need for a second sample collection
• Not recognizing when special crossmatching techniques are required (e.g., for alloimmunized patients)

Remember that while the crossmatch is an important safety measure, it is only one element of the blood transfusion procedure. Correct patient identification at sample collection and at blood administration are equally important safety steps.