Can CT (Computed Tomography) contrast agents interfere with blood draws for tissue typing?

CT Contrast Does Not Interfere with Blood Draws for Tissue Typing

CT contrast agents do not interfere with blood draws for tissue typing when collected immediately after the scan, and there is no need to delay blood collection for this purpose. 1

Understanding CT Contrast and Laboratory Testing

CT contrast agents (typically iodinated compounds) are administered intravenously to enhance visualization of vascular structures and tissues during CT imaging. While these agents can potentially interfere with certain laboratory tests, tissue typing is not among those affected.

Types of Laboratory Interference from CT Contrast:

• CT contrast can interfere with some chemistry tests by causing spectrophotometric interference
• May affect certain endocrine tests and specific assays
• Can cause temporary changes in renal function markers
• May interfere with thyroid function tests due to iodine content 2

Tissue Typing and CT Contrast:

Tissue typing (HLA typing) involves analyzing cell surface proteins and genetic markers, which are not affected by the presence of iodinated contrast in the bloodstream. The analytical methods used for tissue typing (PCR-based methods, flow cytometry) are not susceptible to interference from contrast media.

Timing Considerations for Blood Collection

The European Society of Urogenital Radiology and the European Federation of Laboratory Medicine recommend:

• Ideally, blood samples should be collected before contrast administration when possible
• However, for tests not affected by contrast (including tissue typing), immediate post-contrast collection is acceptable 1

Important Clinical Considerations

When to Be Cautious:

• If collecting blood for thyroid function tests (contrast contains iodine that can affect results)
• If testing for renal function markers immediately after contrast (may show transient changes)
• If performing certain spectrophotometric assays that can be affected by contrast density

Practical Approach:

1. For tissue typing specifically, proceed with blood collection immediately after CT scan
2. No waiting period is necessary between contrast administration and blood collection for tissue typing
3. Document contrast administration in the laboratory requisition for completeness

Conclusion

While CT contrast agents can interfere with some laboratory tests, there is no evidence to suggest they affect tissue typing results. Blood for tissue typing can be safely collected immediately after a contrast-enhanced CT scan without concern for analytical interference.