How to measure NAG

Calibrant Procedure

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The package

Each test pack contains four sets of three bottles; bottle N - a solution of the substrate, bottle A - a mixture of crystaline buffer salts and bottle G - a ready to use solution of colour development buffer, together with two vials of calibrant and a set of instructions (a copy of which can be found at the rear of this booklet). Packs of calibrant (5 vials) can also be obtained separately.

The principle

2-methoxy-4-(2-nitrovinyl) phenyI-2-acetarnido-2-deoxy-(3-D-gluco-pyranoside (MNP-Glc-NAc) is hydrolysed by N-Acetyl-β-D-Glucosaminidase (NAG) with the release of 2-methoxy-4-(2-nitrovinyl) phenol which, on addition of alkaline buffer, produces a colour which can be measured at 505nm.

Figure 3

Reference values: urine

If timed urine samples are to be used for this test the optimum collection period is 24 hours, although such collections may be difficult to administer. It required, a first or timed morning sample, or daytime sample of suitable collection time can also be used. Random samples may be corrected tor urine volume by factoring with the creatinine value for the sample as follows:

In a study of 78 normal individuals (aged 20-59) NAG values ranged between 6.8 and 30.8µmol/h/mmol creatinine (see fig 4). No significant differences were found in the excretion ot NAG in males and females (see fig 5).

Figure 4

Figure 5

Observed NAG values are higher in children under 10 years and, because of the fall in muscle mass, in individuals over 60 years, and should therefore be separately determined. Stable renal transplant patients excrete NAG at a higher rate than in normals.

NAG values: serum

NAG values observed in serum samples are considerably higher than those found in urine: in addition NAG values are known to rise very considerably during pregnancy.

Modifications

Simple modifications enable this assay to be performed on most discrete and centrifugal-type analysers. Instructions are also available to assist isoenzyme studies which can be carried out using the same test packs.

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