Limitations of diagnostic value of the sweat test.

The sweat test, even if carried out by an experienced technician, sometimes 
lacks reproducibility owing presumably to physiological variations (patient's diet, temperature, and 
other factors at present unrecognized). Some patients are particularly prone to 
exhibit this variability and in them a single sweat test is 
almost valueless. The aldosterone status is believed to be responsible for 
a reciprocal relationship between sweat sodium and potassium concentrations: tests done 
on 8 patients show that a high sweat potassium is associated 
with a correspondingly lower sodium--a circumstance which must be borne in 
mind when interpreting a patient's sweat sodium. Of 30 patients presenting 
with a variety of symptoms compatible with a diagnosis of cystic 
fibrosis and with sweat sodium ranging from 50 to 75 mEq/1 
(50-75 mmol/1), only 4 have proved to have cystic fibrosis after 
several years of observations; 13 have later been diagnosed as having 
asthma. The problem of the 'grey area' of uncertainty is aggravated 
by the heterozygous state which is also associated with a sweat 
sodium in this range. Repeated sweat tests are indicated if the 
sweat sodium lies within the 'grey area', and the diagnostic importance 
accorded the test should diminish as the sodium value approaches this 
area. The diagnosis of cystic fibrosis must remain in doubt unless 
there is strong supportive clinical evidence. 