Guanidinoacetic Acid and Creatine, Plasma / Urine

Specimen requirement

Paired plasma and urine samples are required for the investigation of creatine metabolism. All samples should be sent in ice to laboratory as soon as possible.

Plasma: fasting or pre-meal sample

Heparinised blood tube

Volume: 3 mL 
     
Fresh spot urine: First morning void sample

Plain urine bottle

Volume: 5 mL
Authorisation code required No
24 Hr available service No
Patient preparation Plasma and urine creatine may be falsely elevated due to consumption of meat, or due to taking creatine supplement. Abstinence from intake of meat or fish for 24 hours and creatine supplements for 72 hours is recommended before collecting samples for analysis.
Method

Liquid chromatography-tandem mass spectrometry
Reference interval
Guanidinoacetic Acid Age (yrs) Reference Interval
Plasma

< 15

0.35 - 3.1 µmol/L
 

> 15

0.24 - 4.0 µmol/L
Urine

< 15

4 - 220 mmol/moL creatinine
 

>15

3 - 112 mmol/moL creatinine

 

Creatine Age (yrs) Reference Interval
Plasma < 10 1.4 - 109 µmol/L
 

> 10

 4.6 - 98 µmol/L
Urine

< 4

 6 - 1,500 mmol/mol creatinine
  4 - 12

17 - 720 mmol/mol creatinine
 

> 12

 11 - 560 mmol/mol creatinine

 
Clinical indication For screening of inborn errors in creatine biosynthesis or transport including arginine-glycine acyl transferase (AGAT) deficiency, guanidinoacetic acid methyl transferase (GAMT) deficiency and X-linked creatine transporter deficiency (CRTR).
Result interpretation
Condition Plasma Urine
  GAA Creatine GAA / Creatinine Creatine / Creatinine
AGAT Low Low Low Normal
GAMT Elevated Low Elevated Normal
CRTR Normal Normal Normal *Elevated

*inconsistent data (variable urinary creatine/creatinine in females with CRTR deficiency).

Reference:

Sharer DJ et al. Laboratory diagnosis of creatine deficiency syndromes: a technical standard and guideline of the American College of Medical Genetics and Genomics. Genet Med 2017; 19: 256 - 63

Measurement of uncertainty See table.
Frequency of measurement Monthly