The Sural/Radial ratio (SRAR) aids differential diagnosis of length-dependent vs non-length-dependent axonal polyneuropathies.
SRAR = Sural SNAP amplitude / Radial SNAP amplitude
Right (R)
Left (L)
Expected SRAR (adjusted for age and sex)
Formula: expected SRAR = 0.519 − (0.006 × age) + (0.046 × sex)
What is the sural/radial SNAP ratio
The sural/radial ratio compares sural and radial SNAP amplitudes. Low values favor axonal polyneuropathy.
The Sural-Radial Index - widely known by the acronym SRAR (Sural/Radial Amplitude Ratio) - is an electrophysiological biomarker comparing sural nerve (lower limb) SNAP amplitude with superficial radial nerve (upper limb) amplitude. This calculator processes entered values and instantly computes the ratio, aiding differential diagnosis of neuropathies.
1. Pathophysiology and Age Effect (Why Use a Ratio?)
Using a ratio resolves a common lab dilemma: separating physiological aging from nerve disease effects.
The Length-Dependent Rule
In most axonal polyneuropathies (e.g. diabetic polyneuropathy), degeneration affects the longest nerve fibers first. Sural amplitude falls disproportionately faster and more markedly than radial amplitude, which has a much shorter course.
The Aging Factor
Normal aging causes natural loss of myelinated fibers, progressively reducing sensory amplitudes in all nerves. Because this physiological loss affects both nerves globally, their ratio is far less impacted by age than isolated values.
2. The Formula
SRAR = Sural SNAP amplitude (µV) / Radial SNAP amplitude (µV)
3. Result Interpretation
Normal - SRAR ≈ 0.71
Mean value in healthy individuals. Population studies place the 5th percentile (lower limit of normal) around 0.20–0.21.
SRAR < 0.40 - Length-dependent axonal polyneuropathy
More widely accepted abnormality threshold (sensitivity and specificity ~90% for mild axonal PNP - Rutkove 1997). Some studies use stricter cutoffs: < 0.36, < 0.34, or even < 0.21, depending on the cohort.
SRAR preservado with neuropathic symptoms - Ganglionopathy / NLDN
In sensory neuronopathies (ganglionopathies) and CIDP, injury is at the dorsal root ganglion, so long and short fibers degenerate proportionally. SRAR > 0.33 in a patient with severe sensory loss strongly suggests non-length-dependent neuropathy rather than classic polyneuropathy.
4. Limitations
Restricted to Mild Cases
The index was designed to aid diagnosis of axonal polyneuropathies in mild or incipient stages. In moderate/severe stages, sural SNAP is often absent (numerator = 0), and the calculation loses utility for monitoring progression.
Residual Anthropometric Influence
Although age effect is minimized, correlation with age, sex, and height still exists marginally. For advanced laboratories, expected SRAR:
Expected SRAR = 0.519 − [0.006 × age] + [0.046 × sex (M=1, F=0)]
References:
· Rutkove SB et al. Sural/radial amplitude ratio in the diagnosis of mild axonal polyneuropathy. Muscle Nerve. 1997;20(10):1236–1241.
· Overbeek BUH et al. Sural/radial amplitude ratio: reference values in healthy subjects. Muscle Nerve. 2005;32(5):613–618.
· Pourhamidi K. Normative data and quantile regression analysis of the sural-to-radial nerve amplitude ratio. J Clin Neurophysiol. 2025;42(2):145–148. doi: 10.1097/WNP.0000000000001084.
· Pegat A et al. Sural/radial amplitude ratio: a useful tool to diagnose non-length-dependent neuropathy. Muscle Nerve. 2026;73(1):34–40. doi: 10.1002/mus.70046.
Frequently asked questions
What does a low sural/radial ratio indicate?
Suggests axonal polyneuropathy, especially metabolic (e.g. diabetic), with sural predominance.
How to calculate the sural/radial ratio?
Divide sural SNAP amplitude by radial SNAP amplitude in the same limb.