Sweat conductivity and chloride titration for cystic fibrosis diagnosis in 3834 subjects

Abstract

Sweat test is the standard for cystic fibrosis (CF) diagnosis. Conductivity is an alternative method not yet approved, in spite of its good correlation with chloride concentration. The aim was to assess the capacity of sweat conductivity to discriminate between CF and non-CF subjects. Automated measurements of conductivity and chloride concentration were carried out on the same sweat samples from subjects with clinical suspicion of CF. Sweat samples from 3,834 subjects, median age 1.8 years (range 1 month-54 years) were analysed, and those with chloride titration >60 mmol/l were considered as CF patients (n=294). Conductivity median values in CF and non-CF subjects were 111 mmol/l (82-148) and 36 mmol/l (12-89), respectively. The Spearman correlation between chloride titration and conductivity was r=0.60 (P<0.001). The receiver operating characteristics (ROC) curve showed very high agreement between two methods. The best conductivity cut-off value to diagnose CF was > or =90 mmol/l (sensitivity 99.7%, specificity 100%, positive and negative predictive values of 100% and 99.97%, respectively, and kappa=0.998). Likewise, the best conductivity cut-off value to exclude CF was <75 mmol/l. The sweat conductivity method showed good correlation with chloride titration, and accurately discriminated between subjects with and without CF. In accordance with this, CF diagnosis might be confirmed for conductivity values > or =90 mmol/l and excluded for <75 mmol/l. Values between 75 and 89 mmol/l should correspond to an equivocal range. However, more studies are needed to confirm the role of conductivity in definitive CF diagnosis.

Bilateral sweat tests with two different methods as a part of cystic fibrosis newborn screening (CF NBS) protocol and additional quality control.

“…However, this method involves multiple steps for collection and analysis of the sweat sample and involves the risk of volumetric, gravimetric, condensate and evaporation errors in the procedure. In this context, the measurement of sweat conductivity is an increasingly used method because it is easier to perform and requires a smaller sample than QPIT [6].…”

Comparação entre o método clássico de Gibson e Cooke e o teste da condutividade no suor em pacientes com e sem fibrose cística

“…We decided to use a value of 90 mmol/L or higher as the cutoff point for the CF diagnosis based on the study by Lezana et al 20 These authors compared both sweat test …”

“…[14][15][16][17][18][19][20][21] One of these methods includes the use of the device Macroduct ® -a sweat collection system, 22 through which sweat is collected into a plastic coil after stimulation by pilocarpine iontophoresis. Weighting and risk of evaporation are thus eliminated.…”

Sweat testing: can the conductivity analysis take the place of the classic Gibson and Cooke technique?

“…Dessa forma, o valor de concentração derivado é levemente mais alto do que o valor obtido por análise química direta dos íons de cloreto e sódio, já que outros eletrólitos, além do Cl-e do Na+, são inevitavelmente incluídos. Contudo, alguns estudos já foram realizados, demonstrando que há uma correlação íntima e uma razoável concordância (especialmente para os valores mais baixos) entre os valores de equivalentes de NaCl medidos por análise de condutividade e aqueles obtidos pela análise química quantitativa de Cl-e Na+ [5][6][7][8] .…”

“…Dados confirmatórios têm sido fornecidos com relação a precisão, exatidão e reprodutibilidade: com relação à reprodutibilidade, testes repetidos no mesmo indivíduo fornecem resultados semelhantes 7 . Dados confirmatórios também têm sido fornecidos com relação à sua sensibilidade (capacidade de identificar um paciente com CF como tendo a doença) e sua especificidade (capacidade de identificar um indivíduo saudável como sendo saudável) [5][6][7][8][9] . Contudo, há alguma incerteza com relação aos critérios adotados nos 89 0021-7557/10/86-02/89…”