CBG Exam Domains 2026: Complete Guide to All 15 Content Areas

How the ABMGG Biochemical Genetics Blueprint Is Built

Clinical Biochemical Genetics (CBG) certification is administered by the American Board of Medical Genetics and Genomics (ABMGG). Unlike credentials that organize content into a handful of broad practice areas, the biochemical genetics blueprint is organized the way the field itself is organized: by metabolic pathway and disease class. Each of the 15 categories corresponds to a family of inborn errors of metabolism, plus two categories that handle the laboratory side of the discipline and a catch-all for everything else.

This structure shapes how you should study. You are not preparing for a generic genetics exam. You are preparing to recognize a biochemical phenotype, connect it to an enzyme or transporter defect, interpret the analyte pattern that reveals it, and understand the diagnostic, laboratory and management implications. If you are still sorting out what the credential is and who it is for, the overview in What Is CBG Certification? is a good starting point, and CBG Requirements 2026: Eligibility, Prerequisites & How to Qualify covers who may sit for it.

The Weighting Map: Where the Points Concentrate

The published approximate weights are reproduced exactly as supplied. They add up to 102%, a discrepancy that exists in the source itself, so no figure below has been adjusted or normalized.

Two patterns stand out. First, the "big five" (Amino acids, Lipids, Organic acids, Lysosomes, Carbohydrates) account for roughly 62% of the published weighting. Second, the Laboratory domain at 8% is larger than Peroxisomes, Purines and pyrimidines, Transport, Creatine, Neurotransmitters, Metals and Other disease category taken individually. Candidates who treat lab content as an afterthought are leaving a meaningful share of the blueprint under-prepared.

Domains 1-3: Amino Acids, Organic Acids and Cofactors

Domain 1: Amino acids (15%)

The single heaviest domain is also the most classic territory in biochemical genetics. The blueprint subdivides it into specific disorder groups, and each deserves attention to the analyte pattern, not just the name.

Note how the blueprint pairs homocystinuria with re-methylation disorders. That is a cue to understand where the transsulfuration and remethylation pathways diverge, because cofactor-related questions (cobalamin, folate, pyridoxine) bridge Domain 1 and Domain 3. Urea cycle defects also reward careful differential thinking: the position of the block determines which intermediates accumulate.

Domain 2: Organic acids (12%)

Organic acid disorders are interpreted largely through urine organic acid profiles and acylcarnitine patterns, so this domain overlaps heavily with Domain 14 (Laboratory).

The explicit exclusion of cobalamin defects from the methylmalonic acidemia entry is telling: cobalamin disorders are tested under Domain 3, so expect to know how to distinguish an isolated mutase defect from a cobalamin processing defect.

Domain 3: Cofactors (7%)

The Cofactors domain covers cobalamin, biotin, molybdenum, folate, pyridoxine and biopterin, plus an "other" group that includes riboflavin, niacin and thiamine. The unifying exam skill is recognizing that a defect in cofactor metabolism or transport can mimic a primary enzyme defect, and that some of these conditions are treatment-responsive. Biopterin disorders connect directly back to hyperphenylalaninemia in Domain 1, and molybdenum cofactor deficiency connects to sulfite and purine biochemistry.

Domains 4-7: Carbohydrates, Lipids, Lysosomes and Mitochondria

Domain 4: Carbohydrates (10%)

This domain spans glycogenoses (excluding GSD II, which the blueprint routes to the lysosomal domain), disorders of glycolysis, gluconeogenesis (including fructose-1,6-bisphosphatase deficiency), galactose metabolism, fructose metabolism, congenital disorders of glycosylation, and others including the pentose phosphate pathway. Pay attention to the cross-reference: GSD II (Pompe) belongs under Lysosomes, and the blueprint is explicit about that placement. Congenital disorders of glycosylation are a good example of a group where laboratory technique (transferrin glycoform analysis) and clinical phenotype both matter.

Domain 5: Lipids (13%)

Lipids is the second-largest domain and has only three headline groupings, which means each is tested in depth.

Because the blueprint names chain-length categories, expect to reason from an acylcarnitine profile to the chain length of the block. Smith-Lemli-Opitz and Niemann-Pick C also appear here rather than under their more familiar "syndrome" or "storage" headings, a reminder that blueprint placement follows the metabolic pathway.

Domain 6: Lysosomes (12%)

Candidates often find the mucopolysaccharidoses easiest to organize by the glycosaminoglycan accumulated (dermatan, heparan, keratan, chondroitin sulfate) and the urine GAG pattern. The multiple enzyme deficiency group is conceptually different: ML-II and ML-III reflect a trafficking defect (failure of mannose-6-phosphate tagging), so a normal-looking single-enzyme logic will mislead you.

Domain 7: Mitochondria (7%)

Mitochondrial content covers myopathies with laboratory findings, with Leigh syndrome, and with adult presentations; electron transport chain function and assembly; mtDNA mutation disorders; mtDNA maintenance disorders; mtDNA depletion syndromes; and other conditions including MELAS. The domain rewards the ability to separate nuclear-encoded from mitochondrially encoded causes, and to think about heteroplasmy and tissue-specific presentation when interpreting laboratory and molecular results.

Domains 8-13: The Smaller Specialty Categories

Together these six domains carry roughly 15% of the published weighting, but individually they are small enough to be tempting to skip. That would be a mistake, because their content is highly testable and fairly compact.

• Peroxisomes (3%): biogenesis disorders including Zellweger syndrome; single-enzyme disorders including X-linked adrenoleukodystrophy; and others including Refsum and infantile forms. Very-long-chain fatty acid analysis is the signature test here.
• Purines and pyrimidines (3%): purine disorders including Lesch-Nyhan disease and adenosine deaminase deficiency; pyrimidine disorders including UMP synthase deficiency.
• Transport (3%): dibasic aminoaciduria including cystinuria and lysinuric protein intolerance; other amino acid transport disorders; glucose; carnitine; and other.
• Creatine (1%): the smallest domain, with no sub-list published, so treat it as a compact topic to master in a single sitting.
• Neurotransmitters (2%): SSADH deficiency; disorders of dopamine synthesis; and others including GABA metabolism.
• Metals (3%): copper (Wilson disease, Menkes disease, and other), iron, zinc, and other.

Domains 14-15: Laboratory and Other Disease Category

Domain 14: Laboratory (8%)

The Laboratory domain is where biochemical genetics distinguishes itself from a purely clinical genetics exam. The four published subtopics are QA/QC, artifact, techniques, and regulations. In practice, this means being comfortable with how analytes are measured, what can go wrong in specimen collection and processing, how to recognize an artifact that mimics a disease pattern, and the regulatory framework in which clinical biochemical genetic testing is performed.

Laboratory directors and clinical laboratory geneticists are among the professionals for whom this credential is most directly relevant, and their day-to-day work maps closely onto this domain. If you are weighing career paths, CBG Jobs and the CBG Salary Guide 2026: Complete Earnings Analysis cover the employment side.

Domain 15: Other disease category (3%)

This is the catch-all for conditions that do not fit the pathway-based categories. Because the blueprint does not enumerate its contents, the best preparation is broad familiarity with inborn errors of metabolism and a habit of asking "which pathway does this belong to?" when you encounter an unfamiliar disorder.

Sequencing Your Preparation by Domain

Rather than working domain 1 through 15 in numerical order, sequence your study by connectivity. The ordering below front-loads the heavy, interlinked domains and uses the smaller ones as consolidation. For the broader plan, see the CBG Study Guide 2026: How to Pass on Your First Attempt.

Once you have a first pass done, shift toward practice questions that force you to interpret data rather than recite facts. Use the CBG practice test site to check which domains still produce errors, and keep the CBG Cheat Sheet 2026: One-Page Review of Must-Know Facts handy for last-pass review. Our full practice question bank is organized so you can drill the heavier domains first.

Exam Timing and What the Source Does Not Say

The ABMGG has announced that the next biochemical genetics certifying exam is scheduled for August 2027, and biochemical specialties are offered in odd years. The ABMGG has also approved a transition to an annual certifying examination schedule, so check the ABMGG announcements for how that change may affect scheduling. For timing detail, see CBG Exam Dates 2026: Testing Windows, Deadlines & Scheduling.

Several details are intentionally not stated here because they were not established from the official source content supplied: the exact question count, the exam timer, the fee, and the passing score. Rather than guess, confirm each of these directly with the ABMGG. Our related guides on CBG Certification Cost 2026: Complete Pricing Breakdown, CBG Passing Score 2026: Exactly What You Need to Pass, and How Hard Is the CBG Exam? Complete Difficulty Guide 2026 discuss these topics and flag where official numbers should be verified.

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