OBJECTIVES: Many patients undergoing long-term doxycycline treatment do not 
regularly take their treatment because of photosensitivity. Our objective was to 
create an assay for determining doxycycline levels and to use hair samples for 
monitoring the compliance over a longer period of time.
METHODS: We tested sera and hair samples from patients treated with doxycycline 
by a suitable ultra-high performance liquid chromatography (UHPLC) based assay.
RESULTS: We estimated that the speed of hair growth is roughly 1.25 cm per month 
and we were able to determine doxycycline levels over a 6-month period. We 
tested 14 patients treated with doxycycline and we found similar levels of 
doxycycline in the serum and the hair samples representing the last 4 months. 
Linear regression analysis revealed that the level of doxycycline in the serum 
remained stable over time (p = 0.7) but the level of doxycycline in the hair 
decreased significantly over time (p = 0.03) indicating a degradation of this 
molecule in the hair. We detected two patients who did not have antibiotic in 
the hair, indicating a lack of compliance that was also confirmed by interview.
CONCLUSION: Hair samples can be used to test long-term compliance in patients to 
explain failures or relapses.