Quantitative measurement of amino acids in plasma and urine
IEX-Ninhydrin post-column derivitization liquid ion chromatography (detects abnormal amino acid patterns and quantitative analysis)
Urine organic acid analysis by gas chromatography–mass spectrometry
Plasma acylcarnitine analysis by mass spectrometry
Urine purine and pyrimidine analysis by gas chromatography-mass spectrometrySpecific diagnostic tests (or focused screening for a small set of disorders):

Tissue biopsy: liver, muscle, brain, bone marrow
Skin biopsy and fibroblast cultivation for specific enzyme testing
Specific DNA testingA 2015 review reported that even with all these diagnostic tests, there are cases when "biochemical testing, gene sequencing, and enzymatic testing can neither confirm nor rule out an IEM, resulting in the need to rely on the patients clinical course". A 2021 review showed that several neurometabolic disorders converge on common neurochemical mechanisms that interfere with biological mechanisms also considered central in ADHD pathophysiology and treatment. This highlights the importance of close collaboration between health services to avoid clinical overshadowing. Treatment
In the middle of the 20th century the principal treatment for some of the amino acid disorders was restriction of dietary protein and all other care was simply management of complications. In the past twenty years, new medications, enzyme replacement, gene therapy, and organ transplantation have become available and beneficial for many previously untreatable disorders. Some of the more common or promising therapies are listed:

Epidemiology
In a study in British Columbia, the overall incidence of the inborn errors of metabolism were estimated to be 40 per 100,000 live births or 1 in 2,500 births, overall representing more than approximately 15% of single gene disorders in the population.
While a Mexican study established an overall incidence of 3.4: 1000 live newborns and a carrier detection of 6.8:1000 NBS. References
Further reading
Price, Nicholas C; Stevens, Lewis (1996). Principi di enzimologia [Principles of enzymology] (in Italian). A. Delfino. ISBN 978-88-7287-100-3. OCLC 879866185. Mazzucato, Fernando; Giovagnoni, Andrea (2019). Manuale di tecnica, metodologia e anatomia radiografica tradizionali [Manual of traditional radiographic technique, methodology and anatomy] (in Italian). Piccin. ISBN 978-88-299-2959-7. OCLC 1141547603. Torricelli, P; Antonelli, F; Ferorelli, P; Borromeo, I; Shevchenko, A; Lenzi, S; De Martino, A (March 2020). "Oral nutritional supplement prevents weight loss and reduces side effects in patients in advanced lung cancer chemotherapy". Amino Acids. 52 (3): 445–451. doi:10.1007/s00726-020-02822-7. PMID 32034492. S2CID 211053578. External links

Portal of Chemistry (Italian)