Why does sweat taste salty?

"Kiss-and-cradle test" brings clarity

BELO HORIZONTE. In many European countries, as well as the United States and Australia, newborns are routinely screened for cystic fibrosis (CF) using the immunoreactive trypsinogen test (IRT). However, this test alone is not necessarily reliable.

To confirm the diagnosis, children with a corresponding suspicion are given the so-called sweat test. The time-consuming test, which is based on the increased salt content in the sweat of those affected, is still considered the gold standard in cystic fibrosis diagnostics today.

Pilocarpine is applied to the skin, which stimulates perspiration. The sweat is collected over about 30 minutes and then has to be chemically analyzed immediately. It must be carried out twice for a reliable diagnosis.

Preselection for sweat test

Paulo Camargos and colleagues from the Federal University of Minas Gerais in Belo Horizonte, Brazil, are now presenting a diagnostic aid that apparently makes it relatively easy to decide which children need the sweat test (Acta Paediatrica 2015; online March 12).

According to the researchers, this should be advised if parents state that their children's skin tastes salty when kissed and the newborns gain less than 10.5 grams per day.

Camargos and his colleagues had tested their diagnostic aid on participants in a Brazil-wide CF screening program. The children were on average 34 days old.

In all of them, the two-time IRT test carried out as part of the screening had exceeded the specified limit value of 70 ng / ml - there was therefore a suspicion of cystic fibrosis.

Even before the obligatory sweat test, the parents had been presented with questionnaires. Among other things, questions were asked about "abnormalities in the child's sweat" and whether the sweat tasted a bit salty.

In addition, the children were weighed on standardized scales. In the later sweat test, 49 children turned out to be CF-positive: Their sweat chloride concentration was ≥ 60 mmol / l.

The remaining 177 participants tested negative and served as controls.

Many "hits" with the mothers

According to the researchers, it was "remarkable" how reliably even young mothers with low educational status noticed the salty taste of the skin of their CF-positive baby, as it turned out later, in their first child.

42 percent of the mothers of affected children had the right "taste" here. Children whose mothers felt salt were 17 times more likely to be diagnosed with cystic fibrosis than the others.

But the lack of weight gain also provided a clear indication: Babies who put on less than 10.5 grams per day despite a large appetite were five times more likely to get sick.

On this basis, the scientists formed a decision rule: According to the authors, a score of ≥ 1.5 corresponds to a slight weight gain of <10.5 g / day and / or salty-tasting skin.

Here the sensitivity is 61.2 percent and the specificity 89.6 percent. If the score is ≥ 2.5, both symptoms are present at the same time; In this case, the sensitivity drops to 42.9 percent, while the specificity increases to 96.5 percent.

"The diagnosis of CF will also have to be made clinically in the future" - this was what experts had predicted in 2000 (Pediatr Pulmonol 2000; 30: 139-144).

The average chloride content in the sweat of CF babies in the Brazilian study was just under 92 mmol / l - this corresponds to the salt content of seawater and is nine times the concentration in healthy children.

However, it remains questionable whether the decision aid can be used in general, even without prior IRT tests.

In Germany, unlike in Austria, Switzerland, France, Great Britain and the Netherlands, these have not yet been part of newborn screening.