Reliability of result throat swab/sputum

Question

Dear expert team,
I would like to know how reliable are throat swabs of children indeed and also sputum results of adult CF patients. We are having regular throat swabs of our 6-year-old daughter done, as we are cared for in two centers, it deals with different laboratories, sometimes also a swab is done at the pediatrician or the general practitioner.
It is striking to me as well as to the physician in our center, that in one laboratory of one of our centers nearly never a germ is found, often the result is “normal flora of the throat”. In the other laboratories, however, there is always a germ found. I ask myself how this could be. Is it indeed as difficult to identify germs? Even when our daughter had the first finding of Pseudomonas, it had been found in the laboratory that otherwise do not find any germs at all, however at the next test 4 weeks later our daughter was said to be germ-free again. Two weeks later we had a regular appointment at the other center and there the PA was again in the throat swab.
Now one could think, that children are not producing so much secretions, yet and dependent on the place where the germ stays right now, the laboratory can find something. However even in adult CF patients, who have sputum investigated in different laboratories on the same day or within 2 days, there are such differences. How can this be? I am so insecure that I cannot be happy about a germ-free result anymore, because I do not trust it. When doing the swab investigations there is always the hint on CF and on the longer incubation time, therefore I cannot understand how different the results are all the time. Probably you could enlighten this a bit. I would be thankful for it. Many thanks.

Answer

Hello,

the investigation of the microbiology in the airways with the help of a deep throat swab is on the one hand easy and orientating (already in the 80ties proven in literature), however there are some points to take into account!
- In general a deep throat swab is representative for the germ colonization in the airways
- The sputum of course gives a higher degree of security
- The experience of the laboratory does of course play a role, however what is much more important:
- How much time was needed to bring the material from the patient to the laboratory
- Is it indeed a deep throat swab, better is induced sputum after inhalation of hypertonic saline
- Does it deal with a colonization: that means bacteria are lying passively on the surface of the airway mucosa and are eliminated by the body via the natural way or has an infection already started? Clinically proven with increased bronchial reactions. However also slowly an infection can evolve, often more secretion is produced then.
- What does the x-ray show?
- Is there in parallel to the finding of PSA germs also the finding of PSA antibodies? (The meaning of the PSA antibodies has been described in the Journal of Cystic Fibrosis: 13(2014 534-541))
- The finding of a germ or non-finding of a germ gives a hint however no security!
- If one wants to be sure, a bronchoscopy wit a bronchial-lavage would have to be done!
- In order to get real information about the status of the airways the following investigations are necessary: thorax-CT/MRI – lung function – lung clearance index – and also the microbiology! And also the clinical status!
Only a throat swab has therefore not such a great meaning, however the sum of swabs over one year e.g. is giving quite good information. In case of repeated findings – independent of the laboratory – a discussion about a treatment should be held with the CF center.
If the above hints have been taken into account, one can make a secure statement. In general it has to be stated that already in the first years of life marked lung changes have taken place. Therefore an early diagnostic is necessary, that exceeds markedly a throat swab.
The quality and experience of a laboratory alone is not primarily criticized!
These questions, however, can well be clarified with the CF center.

Best regards,
Dr. H.-E. Heuer

07.06.2015