Peripheral neuropathy (length-dependent polyneuropathy)

Fasting laboratory tests

• Fasting plasma glucose
• FBC (with MCV)
• GGT
• AST
• ALT
• Serum creatinine with eGFR
• CRP
• TSH
• Vitamin B12
• Serum protein electrophoresis
• Serum protein immunofixation (straight away if monoclonal gammopathy strongly suspected)if neededThe AAN recommends immunofixation, more sensitive than electrophoresis alone ^[2]. In France, the laboratory can do it on its own initiative if the electrophoresis is abnormal ^[3]: prescribe it from the outset especially for an ataxic sensory-predominant or demyelinating neuropathy, in which an IgM paraprotein is possible ^[4].
• Oral glucose tolerance test (if fasting plasma glucose normal)if needednon-French guidelineDistal sensory polyneuropathy without diabetes proven by fasting plasma glucose: the AAN suggests looking for glucose intolerance ^[1,2].
• HbA1c (if fasting plasma glucose normal)if needednon-French guidelineThe Dutch neurologists' recommendation (NVN, 2019) includes glucose and HbA1c in the baseline tests for any polyneuropathy, diabetes being the leading cause of polyneuropathy in Western populations ^[5].
• Vitamin B6 (if vitamin supplements, vegetarian or vegan diet)if needednon-French guidelineThe Dutch recommendation (NVN, 2019) measures vitamins B1, B6, B12 and folate in the baseline tests for any chronic polyneuropathy, to detect treatable deficiencies and avoidable vitamin B6 toxicity, given the spread of vegetarian or vegan diets and vitamin supplements; it judges this measurement relatively costly but useful ^[5]. The HAS (2007) cites excess vitamin B6 among the toxic causes, without providing for its measurement ^[4].
• Vitamin B1 (if excessive alcohol consumption)if needednon-French guidelineThe Dutch recommendation (NVN, 2019) measures vitamin B1 in the baseline tests for any chronic polyneuropathy and notes that vitamin deficiencies are more frequent in patients who drink too much alcohol ^[5,6]. The HAS (2007) points out that alcoholic neuropathy associated with a vitamin or nutritional deficiency can improve with abstinence and adequate nutrition, without providing for a measurement ^[4].
• HIV serology (if at-risk situation)if neededAt-risk situation or known infection: the HAS has AIDS looked for in the history; HIV causes distal polyneuropathies and mononeuritis multiplex ^[4].
• Hepatitis C serology (if at-risk situation or mononeuritis multiplex)if neededHepatitis or at-risk situation in the history, or mononeuritis multiplex (hepatitis C, cryoglobulinaemia) ^[4].
• Plasma homocysteine, methylmalonic acid and folate (nitrous oxide use)if needednon-French guidelineRecreational use of nitrous oxide, even in the past or initially denied (siphon cartridges, canisters): serum B12 can remain normal despite functional deficiency; homocysteine indicates recent exposure, methylmalonic acid reflects severity, folate worsens hyperhomocysteinaemia ^[7]. Sample as early as possible: these markers normalise quickly after stopping or supplementation; interpret methylmalonic acid according to GFR ^[7].