Background Hydroxychloroquine (HCQ), a 4-aminoquinolone antimalarial, is 
regarded as the oral therapy of choice for cutaneous and systemic lupus 
erythematosus (SLE). It is also licensed for rheumatoid arthritis (RA). Studies 
of HCQ-treated patients with SLE or RA have demonstrated a positive correlation 
between whole-blood HCQ levels and clinical response. Such studies have involved 
measuring whole-blood concentrations at any given time point after HCQ ingestion 
assuming that steady-state concentrations would undergo limited fluctuation over 
a daily interval because HCQ has a long half-life. This approach might not 
sufficiently take into account the potential intra-patient variation in HCQ 
blood levels that can occur over a 24-hour period. Such variation, if 
significant, could affect the credibility of any concentration-response 
relationship provided from these previous studies. Objectives The objectives of 
this report are to: (a) investigate the intra-patient variation in HCQ 
whole-blood levels and (b) suggest an optimum time for sampling patients for 
future studies. Methods Six patients were recruited with cutaneous lupus 
erythematosus who had each been on HCQ 200 mg twice daily for at least six 
months, so that they were at steady-state. Each patient was fasted overnight and 
had standardized meals and dosing schedule. Whole blood was sampled at seven 
time points over 24 hours. Whole-blood HCQ levels were measured with 
high-performance liquid chromatography using gradient elution, fluorimetric 
detection and chloroquine as an internal standard. The assay had a mean inter- 
and intra-day coefficient of variation of 10% and 5% respectively and a limit of 
detection of 5ng/ml. Results HCQ levels appeared to follow a biphasic pattern 
over the sampling period. Maximum levels were noted a median of four hours 
(range 2-6) after ingestion. Median intra-patient variation between trough and 
peak levels, 'Cmax' ((peak - trough)/trough × 100%), was 27% (range 8-150%). 
Conclusions This study demonstrated that whole-blood HCQ levels vary 27% 
(median, range 8-150%) within an individual over a 12-hour period. Drug levels 
might differ between individuals because of multiple factors, including variable 
adherence to medication. Measuring HCQ levels for assessment of drug adherence 
could be valuable in the 'real-world' clinical setting. This could be assessed 
by taking a blood sample at any time following HCQ ingestion. If patients were 
found to have very low or undetectable levels of HCQ, non-adherence to HCQ 
should be suspected.