Phadia’s W1 test is used to detect blood levels of the IgE (antibodies) responsible for allergic reactions (rhinitis, conjunctivitis, asthma) to ragweed (Ambrosia artemisifolia). The interpretation chart accompanying the result expresses the probability of an allergic reaction to ragweed, but not necessarily the severity of the reaction. Case history (symptoms, relation to the season, presence of ragweed in the surroundings, etc.) and clinical examination by the physician are essential for a reliable interpretation of the results. An anti-ragweed IgE result below 0.35 kilounits per litre (< 0.35 kU/L) indicates that ragweed is probably not responsible for the symptoms experienced by the person, but it DOES NOT COMPLETELY RULE OUT THIS POSSIBILITY. All negative and positive results must be interpreted in light of the case history.