If you look for primary aldosteronism, you find it

A condition considered rare is not sought; a condition not sought is not diagnosed; a condition not diagnosed is considered rare…

A diagnostic adage advises that when you hear hoofbeats, you should think “horses”, not “zebras”. This dogma seems to be applied even when standing in the medical equivalent of the African savanna. Accordingly, we may think axiomatically that the default diagnosis should be what is common. Most high blood pressure is idiopathic in nature, and we can fall into the trap of assuming that secondary causes, including primary aldosteronism, are rare and we need not actively look for them.

Australian hypertension guidelines recommend that looking for primary aldosteronism (measuring the plasma aldosterone‐to‐renin ratio) be considered among “additional diagnostics for selected patients” with elevated blood pressure rather than a routine investigation.1 Libianto and colleagues undertook a study of prospective screening in primary care for primary aldosteronism, with the aim of estimating its prevalence among treatment‐naïve general practice patients with high blood pressure.2 They found that the aldosterone‐to‐renin ratio was elevated in one in four patients, for more than half of whom primary aldosteronism was confirmed by further investigations.

Libianto and her colleagues are to be congratulated for their investigation in primary care, where patients with primary aldosteronism are most likely to initially present. One critical factor for good prevalence estimates is representative sampling. A primary care sampling framework cannot be equated with community sampling, but 88% of people in Australia attend general practices over a two‐year period,3 and blood pressure is frequently assessed during their visits, so the authors’ approach was reasonable. People with illnesses are more likely to visit general practices, and the young are less likely to attend and have their blood pressure measured. Consequently, a larger proportion of people visiting general practices have elevated blood pressure than in the general community, and we would also expect the prevalence of primary aldosteronism to be higher.

The participating practices in the study by Libianto and colleagues were a convenience sample, mostly from a relatively affluent area of Melbourne, and neglecting regional, rural, and remote communities where high blood pressure is more prevalent than in cities.4 Further, the patient consent process in a busy clinic with a service delivery model can be problematic. Nevertheless, the reported rate of primary aldosteronism among people with elevated blood pressure (14%; 95% confidence interval, 10–19%) was similar to that found by overseas studies, after taking the enriched sampling in specialist centres into consideration.5

The authors also found that median serum potassium levels were similar in people with primary aldosteronism and those with elevated blood pressure of other cause. But it is diagnostic categorisation (hypokalaemia or normokalaemia) that is important for the clinician, not the central estimate. It would be better to inform the reader that normal potassium levels do not exclude primary aldosteronism.

Libianto and her colleagues appropriately chose newly diagnosed, treatment‐naïve patients for their study, avoiding diagnostic difficulties that would be caused by antihypertensive medications. The benefit of an early diagnosis is that primary aldosteronism may be cured in patients with unilateral adrenal tumours who are willing to undergo surgery. However, in this study this was the case for only two of 256 screened patients. Most patients with primary aldosteronism would continue to be medically managed, but with an aldosterone antagonist‐based strategy rather than a series of unsuccessful trials of other medications and their combinations. That is, the diagnosis facilitates more timely and better blood pressure control for people with primary aldosteronism. On the other hand, considerable medical costs are associated with confirming the diagnosis and with its treatment, and these need to be balanced against the benefits of cure, or appropriate long term medical management must be shown to be superior to usual practice.

Should assessing the aldosterone‐to‐renin ratio be a standard screening test for all patients with elevated blood pressure rather than an additional test for selected patients? Guideline groups base their recommendations on the totality of the evidence. The Australian guidelines are six years old and due for revision, and the study by Libianto and colleagues provides information for the guideline committee to consider.