A method to determine the likelihood that a cystic fibrosis, CF, patient will respond to treatment with an alginate oligomer administered to at least a part of the respiratory system of said patient, said method comprising: (i) obtaining for a test sample of mucus from the respiratory system of the patient one or more normalised infrared, IR, absorbance values at wavenumbers selected from each of the following wavenumber ranges: 1395 cm -1 to 1405 cm -1, 1537 cm -1 to 1547 cm -1 and 1578 cm -1 to 1588 cm -1; or one or more normalised IR transmittance values at each of said wavenumber ranges, and (ii) for each of said wavenumber ranges, providing at least one of said absorbance values from that range or at least one of said transmittance values from that range, thereby obtaining a test pattern comprising absorbance and/or transmittance values at each of the three wavenumber ranges and (iii) comparing the test pattern of absorbance and/or transmittance values so obtained with a negative reference pattern and/or a positive reference pattern, wherein said negative reference pattern is prepared from at least one sample of mucus from the respiratory system of at least one CF patient who did not respond to said treatment, and wherein said positive reference pattern is prepared from at least one sample of mucus from the respiratory system of at least one CF patient who did respond to said treatment, wherein (a) a test pattern which corresponds to said negative reference pattern and/or which does not correspond to said positive reference pattern is indicative that the patient will not respond to said treatment, and (b) a test pattern which corresponds to said positive reference pattern and/or which does not correspond to said negative reference pattern is indicative that the patient will respond to said treatment. (i) obtaining for a test sample of mucus from the respiratory system of the patient one or more normalised infrared, IR, absorbance values at wavenumbers selected from each of the following wavenumber ranges: 1395 cm -1 to 1405 cm -1, 1537 cm -1 to 1547 cm -1 and 1578 cm -1 to 1588 cm -1; or one or more normalised IR transmittance values at each of said wavenumber ranges, and (ii) for each of said wavenumber ranges, providing at least one of said absorbance values from that range or at least one of said transmittance values from that range, thereby obtaining a test pattern comprising absorbance and/or transmittance values at each of the three wavenumber ranges and (iii) comparing the test pattern of absorbance and/or transmittance values so obtained with a negative reference pattern and/or a positive reference pattern, wherein said negative reference pattern is prepared from at least one sample of mucus from the respiratory system of at least one CF patient who did not respond to said treatment, and wherein said positive reference pattern is prepared from at least one sample of mucus from the respiratory system of at least one CF patient who did respond to said treatment, wherein (a) a test pattern which corresponds to said negative reference pattern and/or which does not correspond to said positive reference pattern is indicative that the patient will not respond to said treatment, and (b) a test pattern which corresponds to said positive reference pattern and/or which does not correspond to said negative reference pattern is indicative that the patient will respond to said treatment. (a) a test pattern which corresponds to said negative reference pattern and/or which does not correspond to said positive reference pattern is indicative that the patient will not respond to said treatment, and (b) a test pattern which corresponds to said positive reference pattern and/or which does not correspond to said negative reference pattern is indicative that the patient will respond to said treatment.