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be unreliable in those with more muscle than average for their age, so had requested a referral for another blood test, cystatin C.
Adam’ s cystatin C-based eGFR was normal, and Adam consulted with a nephrologist who confirmed this indicated he had normal renal function and thus no need for invasive testing.
Simon asks his GP if he can have a cystatin C test. He is happy to pay privately for this. The result returns a normal cystatin C-based eGFR of 101mL / min / 1.73m 2. 1
The GP discusses the results with a local nephrologist who concurs that this is indeed indicative of normal renal function. Simon is delighted when the GP informs him that he does not have chronic kidney disease.
Subsequently, Simon and Adam discover three more people in their gym who’ ve been told they have chronic kidney disease based on
their creatinine-based eGFR, and despite being in good health. They all went on to have a cystatin C test confirming normal eGFR and resulting in reversal of their diagnoses of poor renal health.
Discussion
Many GPs and nephrologists are unaware that creatinine-based eGFR has several confounders, in particular muscle mass. As more people use the gym, lift weights and take an interest in developing their musculature, it’ s likely that more otherwise well and fit patients will return
higher than normal serum creatinine levels on routine checkups.
An eGFR based on creatinine is not accurate at the extremes of muscle mass. In frail and sarcopenic patients, a low serum creatinine overestimates GFR, while in fit, gym-going or muscular middle-aged people, it underestimates GFR. In addition, creatine supplements can elevate serum creatinine, as does consumption of meat in the 12 hours before a blood test. Cystatin C testing offers a way to get around these confounders. 2
Cystatin C is a low molecular weight protein produced by all nucleated cells. Its levels are not related to muscle mass or diet. Current guidelines from the international body Kidney Disease: Improving Global Outcomes( KDIGO) recommend using cystatin C in settings where creatinine is known to be unreliable, such as in
If a patient has a low creatinine-based eGFR in the absence of risk factors for CKD( such as diabetes, obesity and hypertension) and has no albuminuria, then it is worth considering a cystatin C test.
people with extremes of body habitus. 3, 4 This test is not MBS subsidised and costs about $ 50.
If a patient has a low creatinine-based eGFR
in the absence of risk factors for CKD( such as diabetes, obesity and hypertension) and has no albuminuria, then it is worth considering a cystatin C test. The KDIGO guidelines also recommend it for patients consuming keto, vegan or vegetarian diets, for patients with sarcopenia, and for those engaging in extreme sport, exercise, or body building. 3
References on request from kate. kelso @ adg. com. au
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An eGFR based on creatinine is not accurate at the extremes of muscle mass.
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ANSWER The answer is a. Exercise-induced vasculitis— also known as golfer’ s vasculitis, Disney rash and jogger’ s petechiae— is a benign cutaneous small-vessel vasculitis. 1, 2
It is an inflammatory neutrophilic disorder involving the small- or medium-sized blood vessels of the skin and subcutaneous tissue. The underlying pathophysiology is not fully understood and is likely multifactorial, including genetic factors, impaired thermoregulation, arterial remodelling and immune system activation. 1
The typical presentation is with a petechial rash on the legs following physical triggers— usually a combination of heat exposure and prolonged exercise, including but not limited to hiking, golfing, jogging and walking. Characteristically, the rash spares skin that has been covered— for example, by socks or stockings. The condition predominantly affects women in their 50s who have no significant medical history. The rash may be painless or associated with itch, discomfort and burning sensation. It may recur with subsequent exposure to prolonged exercise in heat. 1, 2
Exercise-induced vasculitis may be diagnosed clinically if typical features are present and in the absence of any evidence of systemic disease. However, the differential diagnosis is broad and includes other forms of systemic and cutaneous vasculitis, stasis dermatitis, cellulitis and pigmented purpuric dermatoses. If there is any diagnostic uncertainty or concern, blood tests for FBC; inflammatory, vasculitis and autoimmune markers; formal urine assessment; and skin biopsy may be indicated. Biopsy will demonstrate a leukocytoclastic vasculitis, and direct immunofluorescence may demonstrate C3 and immunoglobulin M deposits in vessel walls. Investigations will be otherwise normal. 1, 2
Exercise-induced vasculitis is usually self-limiting, with resolution within 10-14 days. Management is supportive and symptomatic, with simple analgesia, limb elevation, cold compression and heat avoidance until the rash resolves. Patients with recurrent episodes may need to be advised to avoid exercising in the heat.
In this case, the appearance, limited distribution— to exposed lower legs only— and lack of pruritus / discomfort make irritant contact dermatitis less likely. Idiopathic thrombocytopenia is more likely to be associated with a history of easy bruising and / or bleeding. The lack of any systemic features favours a benign cause rather than meningococcaemia.
Dr Kate Kelso is a GP and medical editor at Australian Doctor. References on request from kate. kelso @ adg. com. au
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