Aarsand AK1, Petersen PH2, Sandberg S1
1Norwegian Porphyria Centre (NAPOS), Laboratory of Clinical Biochemistry, Haukeland University Hospital, Bergen, Norway, 2Department of Clinical Biochemistry, Odense University Hospital, Odense, Denmark
Many patients with acute intermittent porphyria (AIP) have a baseline excretion of porphobilinogen (PBG) 10-20 fold the upper reference value even when in remission. In order to distinguish the natural variation of porphyrin excretion from that which is assumed to accompany an acute attack, it is thus important to know the within-subject biological variation of porphyrins and porphyrin precursors in urine.
Fifteen AIP patients without symptoms for the past two years were included. Short-term variation was calculated based on urine samples collected for ten consecutive weeks whereas the long-term variation was calculated based on samples collected through a two year period.
The short-term within-subject biological coefficients of variation (CVBw) of urinary ALA, PBG and total porphyrins per creatinine for latent AIP are 18%, 20% and 28% respectively.
Calculating the level of ALA, PBG and total porphyrins per mmol creatinine significantly reduces the within-subject biological variation. The long-term CVBw of 25.1% for PBG is significantly larger than the short-term, whereas the long-term and short-term CVBw for ALA and total porphyrins are not significantly different. This means that when using the level of urinary PBG in the assessment of AIP-related symptoms, not only must it be compared to the patient’s level of urinary PBG when in remission, the time lapse since the previous urine sample must also be taken into consideration.