Investigations
1st investigations to order
CRP
Test
CRP is an acute phase protein that is elevated in most inflammatory conditions. A normal CRP level would argue against a diagnosis of polyarteritis nodosa (PAN).
Result
elevated
erythrocyte sedimentation rate (ESR)
Test
ESR is raised in inflammatory conditions, including PAN.
Result
elevated
FBC
Test
A normochromic, normocytic anaemia is a common feature of inflammation and, therefore, a frequent finding in PAN. Neutrophilia or eosinophilia may be present. A raised platelet count is a recognised feature. In contrast, a low platelet count may indicate other disease processes such as systemic lupus erythmatosus (SLE) or catastrophic antiphospholipid syndrome.
Result
normocytic anaemia, mildly elevated WBC count, raised platelet count
complement
Test
Activation of the complement cascade by immune complexes (especially in hepatitis B virus [HBV]-related PAN) may result in low complement levels.
Result
reduced
serum creatinine
Test
PAN can affect the renal arteries and cause renal ischaemia or infarction with a resulting rise in serum creatinine.
Result
elevated or normal
midstream urine analysis
Test
Urinalysis is useful to rule out infection and exclude glomerulonephritis. PAN does not cause glomerulonephritis, and the presence of red cell casts or severe proteinuria suggests an alternative diagnosis such as granulomatosis with polyangiitis (GPA, formerly known as Wegener's granulomatosis) or microscopic polyangiitis (MPA).
Result
mild proteinuria or normal
liver function tests
Test
Mild elevation of liver enzymes is common. Acute hepatitis is possible during initial infection with HBV or may be a result of ischaemic hepatitis.
Result
elevated liver enzymes
HBV serology
Test
HBV infection typically occurs a few months before the development of HBV-related PAN.
Result
HbsAg positive and/or HbeAg positive
hepatitis C virus (HCV) serology
Test
HCV infection is associated with a predominantly cutaneous form of PAN.
Result
positive anti-hepatitis C antibodies
cryoglobulins
Test
Presence of cryoglobulins would suggest an alternative diagnosis. Cryoglobulins are strongly associated with hepatitis C virus infection and can cause a small-vessel vasculitis.
Result
no cryoglobulins
blood culture
Test
It is important to exclude endovascular infection, which can mimic vasculitis, especially before embarking on immunosuppressive therapy.
Result
no growth of organisms
creatine kinase
Test
Creatine kinase is not markedly elevated in PAN, even when there is muscle involvement.
Result
normal or mildly elevated
anti-neutrophil cytoplasmic antibodies (ANCA)
Test
PAN is not associated with ANCA. A positive result in the context of suspected vasculitis would suggest an alternative type of vasculitis.
Result
negative
antinuclear antibodies (ANA)
Test
Can be a useful test in the right context. A positive ANA may help to point towards an alternative diagnosis such as SLE or another connective tissue disorder.
Result
negative
anti-double-stranded DNA antibodies (anti-dsDNA)
Test
A positive result for anti-dsDNA can be useful to make a diagnosis of SLE if manifestations fit with this diagnosis.
Result
negative
rheumatoid factor
Test
Useful only if rheumatoid arthritis is a possible alternative diagnosis on the basis of the history and examination.
Result
negative
antibodies to cyclic citrullinated peptides (anti-CCP antibodies)
Test
Useful only if rheumatoid arthritis is a possible alternative diagnosis based on the history and examination.
Result
negative
lupus anticoagulant
Test
May be present in antiphospholipid syndrome.
Result
negative
immunoglobulin G antiphospholipid antibodies
Test
May be present in antiphospholipid syndrome.
Result
negative
B2 glycoprotein
Test
May be present in antiphospholipid syndrome.
Result
negative
fibrinogen
Test
Fibrinogen may be elevated in PAN as a marker of acute inflammation. It may be low in catastrophic antiphospholipid syndrome.
Result
normal or elevated
conventional digital subtraction angiography
Test
This is the standard method of imaging potentially affected vessels. The sensitivity and specificity of the test in a selected population suspected of vasculitis is 89% and 90%, respectively.[40] The main disadvantage is that it is a relatively invasive procedure.
Result
microaneurysms, vessel ectasia, or focal occlusive lesions in medium-sized vessels
echocardiography
Test
Useful to exclude an alternative diagnosis such as endocarditis, atrial myxoma, or a left ventricular thrombus.
Result
normal
Investigations to consider
magnetic resonance angiography (MRA)
Test
MRA is a non-invasive method of imaging potentially affected vessels, but it has lower spatial resolution than conventional angiography and is not very good at detecting microaneurysms.[41] It has the benefit of being able to show areas of organ infarction.
In patients with normal MRA and suspected PAN, conventional angiography is indicated.
Result
aneurysms or focal occlusive lesions of medium-sized vessels
CT angiography
Test
CT angiography is a non-invasive alternative to MRA and conventional angiography. It has better spatial resolution than MRA, but it is not as good as conventional angiography at detecting microaneurysms.[41] It has the benefit of being able to show areas of organ infarction.
In patients with normal CT angiography and suspected PAN, conventional angiography is indicated.
Result
aneurysms or focal occlusive lesions of medium-sized vessels
biopsy of affected tissue
Test
The tissue sampled must be targeted from the history and examination. Muscle, peripheral nerves, kidney, testis, and rectum, when involved, provide the best targets. A positive skin biopsy does not always suggest systemic involvement.[Figure caption and citation for the preceding image starts]: Pathology specimen of a medium-sized muscular artery showing transmural inflammationFrom the collection of Dr Raashid Luqmani [Citation ends].
[Figure caption and citation for the preceding image starts]: Biopsy specimen showing florid transmural inflammation of a small arteryFrom the collection of Dr Loic Guillevin [Citation ends].
Result
focal and segmental transmural necrotising inflammation in a medium-sized vessel (i.e., a small- or medium-sized artery)
HIV serology
Test
It is important to rule out HIV infection, which is a cause of vasculitis in its own right.
Result
negative
genetic test for adenosine deaminase 2 (ADA2) deficiency
Test
Consider testing for lack of function mutations in the adenosine deaminase 2 gene (previously called the CECR1 gene) in young patients presenting with classical features of PAN, especially if stroke is part of the presentation.
Result
The test is negative in idiopathic PAN. However deficiency of adenosine deaminase 2 (DADA2) is a recently described monogenic vasculitis syndrome and some patients previously diagnosed with PAN likely had DADA2.
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