Both we at King's Mill and Nottingham City use reference ranges
derived from hypertensive subjects, originally collected by Jean Wardell I
think. We display both normotensive and hypertensive ranges on our reports
with an explanatory note. But, of course, we still get a number of results
above the hypertensive range that do not appear to lead to a diagnosis of
phaeochromocytoma(not more than 3 x the upper limit of the normotensive
reference range).Perhaps it's not surprising - the condition has such a low
prevalence that test specificity needs to be very high to avoid a
significant number of false positives. Drug interference and pharmacological
effects of hypertensive agents contribute to the non-specificity.
A high proportion of such patients (presumably those with a low clinical
suspicion) are not followed up by imaging investigations.We are auditing
catecholamine reference ranges in Trent via a questionnaire this year.As
part of that exercise I shall be looking for large studies with ROC
analysis. Any contributions welcome.
A big retrospective study in Auckland in 1997 reported that 10% of patients
collecting urine for exclusion of phaeochromocytoma showed at least one
'raised' urine free catecholamine (normotensive reference range) while their
pick-up rate for phaeos was 0.1%(N Z Med J 1997 Sep 12; 110 (1051):331-3)
Our physician colleagues need to understand that in this situation, and
particularly if an individual patient has a low clinical index of suspicion,
a result less than twice the upper limit of the normotensive reference range
has a low predictive value for phaeochromocytoma.
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