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Newborn screening and sweat test

Dear expert team,
my daughter (18 month old) suffers from her 15th week of life on of chronic productive cough of unknown origin. Three weeks after the cough had started at that time, she got a long lasting bronchitis (RSV negative April 2013), that did not resolve.
With the inhalation of beclomethasone the mucous congestion became less, however the productive cough stayed. When she was 9 months old, we have been sent to have a sweat test done. It was negative, the values were within the normal range. Furthermore, she was not conspicious in the newborn screening, that had been done after birth. No allergies could be detected. Thrombocytopenia of unknown origin, no spitting child, throat swab without pathological findings. No typical recurrent obstructive bronchitis, thorax x-ray with reaction of the hilus, otherwise without pathological findings, auscultation of the lungs also without pathological findings. They say the secretions are audible "more in the upper parts, not in the lungs", no constant rhinitis. Now we have been sent to genetics, as in the meantime a developmental retardation has shown up (BMI 14, in spite of good appetite and retardation of growth). The geneticist was however now of the opinion, that such an investigation of blood would be rather supportive however could not soleyly be used to make the diagnosis of CF. We are quite desperate, as either the pediatrician nor the pulmonologist can make a diagnosis. Now we are told to go to hospital for bronchoscopy. We are afraid of this, i.a. because of the thrombocytopenia.
We inhale 2-3 times a day with NaCl + salbutamol or hypertonic saline, in order the secretion become loose and she can cough it up. In the mornings after the first deep breath she has dry cough and after inhaling, it becomes loose. Often then it is calm for some hours. For 10 months she is getting fluticasone. There was a time period of about 2 months in November and December, when she had no cough for 2-3 weeks with a daily dosage of 250 micrograms fluticasone, also no wheezing and rattling. After reduction, all started again. A cure at the Baltic sea did not have any effect.
Are there cases of CF, that can not be detected via the sweat test and the newborn screening (IRT/PAP)? I am very despaired, as nobody knows what she has and how one can help her and which perspective comes about.
I would be very pleased about a short judgement from your side.
you report that your daughter, who is 18 month old in the meantime, suffers since her 15th week of life from a chronic, productive cough and inhales actually regularly with NaCl and salbutamol or with hypertonic saline and is treated additionally for 10 months with fluticasone, without that the cough had disappeared. The newborn screening for CF as well as the sweat test had been negative. A thrombocytopenia had been diagnosed and a developmental retardation of your daughter occurred with a BMI of 14 and a growth retardation.
In the meantime you have been sent to a human geneticist, who has told you that via a molecular genetic investigation the illness of CF could not be diagnosed nor excluded for sure in this case.
You ask, if with the existing findings the illness of CF could still be underlying and you are worrying due to the thrombocytopenia.
About the situation of your daughter it can be said, that the picture of results is very complex and that one needs furhter information for comprehensive advice.
First of all it has to be questioned, if your daughter has been investigated in detail in a hematologic department because of the thrombocytopenia (how high are the thrombocytes?) and the chronic lung manifestaion and if hereby the function of the granulocytes has also been investigated.
Furthermore it becomes not clear from your information, if the function of your daughter's pancreas has been investigated also in the end, i.e. after occurrence of the developmental retardation. Other causes have to be considered and excluded like e.g. the seldom Shwachman-Diamond-syndrome and other causes like celiac disease.
To the question, if in spite of the existing results a CF could be underlying, it has to be said, that it is indeed possible, as not all CF patients are caught by the screening and in some patients the sweat test can be between 30-40 mmol/l chloride concentration. In these cases, a molecular genetic investigation does not help in general, like the geneticist told you, but other physiological investiagtions of the cell are necessary, that can be explained by the physicians of your CF center, in order to calrify the situation.
With a great degree of security, your daughter is not suffering from CF.
A detailed investigation by an experienced pediatric hematologist, infectiologist and gastro-enterologist is necessary in order to make the right diagnosis together.
We wish you and your daughter all the best.

Yours sincerely,
Dr. H.-G. Posselt