[Induced sputum in asthma: study of validity and repeatability]

To assess the validity and reproducibility of determining total and differential cell (DC) counts and of eosinophilic cationic protein (EPC) levels in induced sputum. Clinical analysis of validity and reliability of a measurement tool. Twenty-one asthmatics [age: 31(14) years; FEV1: 78(21)% of reference value; PC20FEV1: 0.02(1.1)mg/ml] and 10 healthy subjects [age: 30(3) years, FEV1: 99(12)% of reference value, PC20FEV1: > 8(3.7)mg/ml]. A sputum sample was collected from each individual using hypertonic saline. Sputum was separated from saliva and then divided into two portions as uncontaminated as possible by squamous cells. Both portions were treated independently and simultaneously within two hours with a solution of bithiothreitol to disperse the cells, and then centrifuged. The supernatant was poured off and frozen al -70 degrees for later determination of EPC. The sediment was stained with Papanicolaou stain, toluidine blue and eosinhematoxylin. DC count was expressed as a percentage of total inflammatory cells. ECP was determined with a CAP-System and a commercial kit (Pharmacia Diagnostics SA, Uppsala, Sweden). Two subjects in each group were unable to produce valid sputum samples (20% of healthy subjects and 10% of asthmatics). The validity of the method was demonstrated by significant differences between asthmatics and healthy subjects in both eosinophil and EPC levels. The reproducibility of method was verified by comparing the results for the sputum fractions; no significant differences were found in cell counts or EPC levels. The within-group correlation coefficients for the sputum fractions ranges between 48 and 77% for all cell types, except squamous cells, which gave a coefficient of 18%. For EPC the correlation coefficient was 98%. DC and EPC determinations in sputum induced by hypertonic saline are valid and reproducible.