Limitations of diagnostic value of the sweat test

Repeatability of standardised nasal nitric oxide measurements in healthy and asthmatic adults and children

Several nasal nitric oxide (NO) measurement techniques have been described, but there is not a widespread measurement technique for measurement of nasal NO. In this study we evaluated the repeatability of one technique of nasal NO measurements using the nasal application of the NIOX system.

METHODS

Three nasal NO measurements (NIOX Nitric Oxide Monitoring System, Aerocrine, Sweden) were made on a single occasion in 22 healthy subjects and 27 patients with asthma, aged 5-69 years. Nasal NO was sampled during breath hold from one nostril at a flow rate of 5 ml/s.

RESULTS

The repeatability of nasal NO assessed by the coefficient of variation (CV) was 12.5% (95% Confidence interval (CI) 11.0-14.7%) for the total population. Healthy adults had a significantly better repeatability than healthy children (P<0.008). The mean NO level for the total population was 837 ppb. The mean NO levels in children were lower than in the adults (751 and 897 ppb, respectively). The mean breath hold length needed to obtain a steady NO plateau for all subject categories combined was 20.4+/-6.01 s. The average number of attempts needed to obtain three approved NO measurements was 5.4 (range 3-13) for the total population. There were no significant differences between the different subject categories. When using two measurements per session instead of three, the overall CV was 10.5% (95% CI 8.8-13.1%). Most subjects found the measurements easy to perform and all of them would accept to do the examination as a routine.

CONCLUSIONS

Nasal NO measurements in NIOX may provide a useful reliable clinical tool to assess and monitor upper airways in different diseases, for example PCD and rhinitis, and are acceptable by both healthy and asthmatic adults and children, as a part of their routine visit to a physician.

Borderline sweat test: criteria for cystic fibrosis diagnosis

The CF diagnosis can be difficult in subjects with disease consistent symptoms and borderline values of the sweat test. This study aimed to evaluate possibly resolutory criteria. Seventy-one ill subjects with borderline sweat test values (40-70 mEq/l Cl-) aged 0.1-37.7 years (mean, 8.7; SD, 7.3) were studied and compared with 33 age-matched CF patients (Cl- over 75 mEq/l) and 25 healthy subjects. A complex CF score based on 25 variables was set: CF-specific clinical conditions, detailed analysis of sweat test (salt-free diet test included), pancreatic function, Pseudomonas infection, and others. The score ranged from 0 to a maximum of 65 and clearly distinguished the CF patients from the healthy subjects. Therefore the CF score was assumed as the best reference criterion to define the diagnosis in the borderline group: 27 of them (38%) were assigned to the CF condition and 44 (62%) to the healthy one, with a quite clear separation between the 2 subgroups. With respect to these assignments, the most discriminant variable turned out to be the sweat Cl- persisting above 40 mEq/l after 5 days of salt-free diet (mean error, 18.4%). Neither sweat Cl-/Na+ ratio nor bicarbonate duodenal output showed better discriminant power (mean error, 29% and 25%, respectively). It is concluded that in borderline situations of sweat test results a definite CF diagnosis can be achieved only by compounding many clinical and laboratory data with a preference for the sweat test after a salt-free diet period. The CF score proposal can be an effective help.

Cystic fibrosis in Jordan: a pilot study

Cystic fibrosis (CF) was diagnosed in 12 children from a total of 15,314 children admitted to the Paediatric Department at Jordan University Hospital (JUH) over the period 1976-1980. A diagnosis of meconium ileus was made in five children. The remaining seven affected children were diagnosed clinically on the basis of recurrent chest problems, failure to thrive, positive family history and a high level of sweat chloride, range 72-115 mEq/l (mean 97.6 mEq/l). Five children died (41.7%); three of the group with meconium ileus died during the post-operative period and two of the other group died soon after the diagnosis was made. The cause of such a high mortality rate is multifactorial. As CF was only recently recognized in Jordan, it is believed that the prevalence of CF among Jordanian children is underestimated. CF is still a very serious illness that demands early diagnosis and an intensive programme of continuing care. Therefore, further prospective study to evaluate its prevalence and impact on child health in Jordan is urgently needed.

Overdiagnosis of cystic fibrosis

In 7 patients seen over 3 years, cystic fibrosis had been wrongly diagnosed. The initial sweat test was misleadingly high in only 3 cases. In 1 case no sweat test had been done, and in 4 one or more normal sweat-test results were ignored. As a result of misdiagnosis 4 children were sent to schools for the physically handicapped and 1 man lost his job in the police. Despite warnings about the limitations of sweat tests and dangers of diagnosing cystic fibrosis without typical clinical features, cystic fibrosis is wrongly diagnosed in substantial numbers of children in England.

Does that child really have cystic fibrosis?

A diagnosis of cystic fibrosis was incorrectly made after false-positive sweat tests in 14 children. 13 of these children had been tested at hospitals where it seems likely that sweat tests were not done very often. All the children had normal sweat-electrolyte values when the test was repeated at a regional paediatric centre where approximately 250 sweat tests are done each year. In 5 cases, detailed testing of pancreatic function was normal. None of the children had typical chest disease and only 2 had gastrointestinal symptoms. In the absence of the typical clinical features of the disease a diagnosis of cystic fibrosis should be made with extreme caution and only after meticulous testing of both sweat electrolytes and pancreatic function.