C. Persistent elevation of serum creatinine to 4 mg per deciliter (dL) (100 ml) or greater or reduction of creatinine clearance to 20 ml per minute or less, over at least 3 months, with one of the following:

2. Persistent motor or sensory neuropathy (see 6.00E4);

Blood and urine tests required for this listing must be documented in the medical records on more than one occasion over a period of at least three months.

Creatinine - normal product of muscle metabolism.
Serum creatinine - amount of creatinine in the blood, which measures renal function; determined by a blood sample
Creatinine clearance test - test for renal function based on the rate at which creatinine is excreted by the kidney; determined by a blood sample with a 24 hour urine sample

A specific assessment of an individual's overall residual function cannot be directly correlated with a set abnormality of serum creatinine or creatinine clearance alone. An individual with a serum creatinine less than 4 mg/dl may not have significant loss of function; however, as the creatinine begins to exceed 4 mg/dl, complications of chronic kidney disease and loss of the individual’s overall level of function increases.

Neuropathy - a problem in peripheral nerve function (any part of the nervous system except the brain and spinal cord) that causes pain, numbness, tingling, and muscle weakness.
Sensory neuropathy - a neuropathy or polyneuropathy (more than one nerve) involving only the sensory nerves (nerves of feeling)
Motor neuropathy - a neuropathy or polyneuropathy (more than one nerve) involving only the motor nerves (nerves of muscle function)

Medical evidence should document a sensory and/or peripheral neuropathy on physical exam (decreased sensation and or muscle weakness), which might be supported by studies such as EMG/NCS (electromyography/nerve conduction study).

The preamble specifies the longitudinal clinical record must show the neuropathy is a “severe” impairment that has lasted or can be expected to last for a continuous period of at least 12 months. This means it must be “more than a slight abnormality, and has more than a minimal effect on the ability to do basic physical or mental work activities.”
This gives a lot of leeway here because by this definition, the neuropathy does not have to produce motor loss or a gait impairment. There could just be significant enough pain that interferes with the ability to function and work.