Fast Facts: Specific IgE vs CAST
Authors: Dr Carla van Heerden & Tayla Matthews
Date: September 2025
DNA Testing Overview
Next-generation sequencing (NGS) of nuclear DNA provides size- and sequence-based information for a variety of identity-informative markers. It is suitable for both pristine and compromised or degraded DNA samples and is commonly used in criminal investigations.
NGS of mitochondrial DNA (mtDNA) is an alternative identification method when nuclear DNA is absent or too degraded. mtDNA analysis offers maternal lineage-based genetic information, useful for identifying both nuclear family members (e.g., mother and child) and extended family (e.g., grandmother, maternal cousins).
Allergy Testing Methods
Specific IgE Testing (formerly known as RAST)
- Detects IgE antibody-mediated allergies.
- Suitable for identifying immediate allergic reactions.
- Lower cost compared to other methods.
- Requires a serum SST (yellow top) tube with a minimum volume of 5ml.
- Specimens are tested daily.
CAST (Cellular Allergen Stimulation Test)
- Detects basophil-mediated allergies to inhalants, food, additives, preservatives, medications, and occupational allergens.
- Can identify both immediate and delayed allergic reactions.
- Higher cost.
- Requires a heparin (green top) tube with a minimum volume of 4ml.
- Testing should be done at least two weeks after a reaction and during a systemic steroid-free period.
- Samples must be collected from Sunday to Thursday to ensure delivery to the lab within 24 hours.
- Testing is not performed on weekends or public holidays.
CAST for Medication Allergy Diagnosis
CAST is especially valuable for diagnosing suspected allergies to medications:
- Penicillin CAST: Includes co-amoxy clavulanic acid, penicillin metabolites, and penicillin ring structures.
- NSAIDs CAST: Includes aspirin, ibuprofen, and diclofenac.
- Paracetamol CAST
- Diverse CAST: Used for testing medications not available as commercial allergens.
- Medication (tablet, capsule, or liquid form) should accompany the blood sample and be clearly indicated on the request form.
- Inhalers (gas), oils, and ointments are not suitable for testing.
Contact Information
For more details, contact the Immunology Department:
Phone: 012 678 0613/14
🌐 ampath.co.za