Guides › Typing methods and resolution

HLA typing resolution explained, from allele group to full sequence

Updated 2026-10-29 · allele examples checked against IPD-IMGT/HLA 3.66.0 on 2026-10-29

HLA typing resolution is how precisely a result pins down which alleles a person carries. It ranges from the allele group (the first field of the name) to the complete sequence of the gene (all four fields). Most of the confusion around resolution comes from treating "how many fields the reported name has" as if it were "how much of the gene was examined". They are related, but they are not the same thing.

This guide uses the field definitions from the WHO nomenclature at hla.alleles.org, and the counts are checked against IPD-IMGT/HLA release 3.66.0.

What each level distinguishes

Each additional field of an allele name separates alleles by a finer kind of difference:

A harmonization working group of clinical, registry and histocompatibility organizations published shared definitions of typing-resolution terms in 2011 (Nunes and colleagues, Blood and Human Immunology). If you need formal definitions of terms like "high resolution" for a protocol, use that document rather than informal usage.

How much ambiguity each level leaves

A result at a given level is consistent with every named allele below it. In release 3.66.0:

Two-field resolution is enough to fix the protein, and for many questions the protein is what matters. But "two-field" does not mean "one allele". It means one protein and possibly hundreds of sequences.

The same holds at other loci. In release 3.66.0, B*07:02 covers 255 named alleles, DRB1*15:01 covers 89, and DQB1*06:02 covers 120. A few two-field names are much smaller: A*01:02 covers 4.

G and P group reporting

Many typing strategies focus on the exons that encode the peptide-binding domains. The 2010 nomenclature update said so directly: typing strategies are frequently aimed at resolving alleles that differ within the peptide-binding domains, but fail to exclude alleles that differ elsewhere.

G and P codes were introduced in 2010 to report exactly that situation:

• A G group (for example A*02:01:01G) is the set of alleles with identical nucleotide sequence in exons 2 and 3 for class I, or exon 2 for class II.
• A P group (for example A*02:01P) is the set of alleles that encode identical protein in those exons.

A G-group result is therefore a statement about a defined region of the gene, not about the whole allele. It cuts across second fields: A*02:01:01G includes A*02:66 and A*02:75 as well as A*02:01 alleles, and it includes null alleles such as A*02:43N. The G and P groups guide covers this in detail.

Why name length is not resolution

Three common situations break the assumption that a longer name means a better typing.

Some alleles have short names. Every allele gets at least two fields, and longer names are only assigned when a relative requires them. An allele with no synonymous or non-coding relatives can be fully described by a two-field name. A*03:06 is an assigned allele in release 3.66.0 with no third field. A report of A*03:06 names one specific allele, while a report of A*02:01 covers 519.

A name does not say what was examined. G and P codes exist because typing aimed at the peptide-binding exons leaves alleles that differ elsewhere unresolved. Whatever name ends up in a report, only the method and the list of alleles the result is consistent with tell you what was actually resolved.

Names change. When a new relative is named, an existing allele gains a field. In release 3.66.0, A*01:284 became A*01:284:01. The resolution of the original typing did not change; the name did.

So, when you store or exchange results, keep three things separate: the reported string, the method or region examined, and the full list of alleles the result is consistent with, against a stated release. A GL String can carry that list.

Shorthand that lowers resolution

Several reporting shorthands look like allele names but carry less information than their shape suggests:

• The XX code, as in A*02:XX, stands for the whole first-field group: 2,127 alleles in release 3.66.0.
• Multiple allele codes, such as A*02:AB, are reporting shorthand for a list of alleles. The 2013 GL String paper (Milius and colleagues) describes them as allele codes developed by NMDP and says their use today increases ambiguity. Expanding one needs the code definitions.

None of these should be truncated, padded or silently converted. A field of XX is not a wildcard that can be replaced with 01.

Resolution in matching

Resolution matters most when two typings are compared. The 2007 Lee study in Blood, of 3,857 unrelated-donor transplants, defined its 8/8 match as high-resolution matching at HLA-A, -B, -C and -DRB1, and reported that this was the minimum level of matching associated with the highest survival in that cohort. That study is about outcomes; this guide is only about what the typing can show.

The arithmetic point is simpler: two results can only be called a match or a mismatch at the resolution both of them reach. If one side is typed only to A*02, it cannot be counted as matching an A*02:01 on the other side. The HLA-Verify match endpoint reports such a locus as potential, raises resolution_insufficient, and leaves it out of the count rather than guessing. The 8/8 and 10/10 guide walks through an example.

Checking the resolution of a name

GET /v1/allele/{name} reports what a name covers in the pinned release. For a valid prefix such as A*02:01 it returns the member count, a sample of members and a resolution block giving the field count, the level (allele_group, protein or coding_sequence) and what that level does and does not distinguish. For a group name it returns the group type and member count.

Frequently asked questions

Is two-field typing high resolution?

Two fields fix the protein sequence. The term "high resolution" has formal definitions in the 2011 harmonization paper by Nunes and colleagues; check those, and your own protocol, rather than relying on field count alone.

Does a four-field name mean the whole gene was sequenced?

Not necessarily. A four-field name identifies one fully named allele, but the name alone does not record which parts of the gene the method examined. The method and the list of alleles the result is consistent with tell you what was actually resolved.

Why do some alleles only have two fields?

Names are only as long as needed. If no relative differs from an allele only synonymously or outside the coding sequence, a two-field name identifies it uniquely.

What does A*02:XX mean?

It stands for the whole A*02 allele group, which covers 2,127 named alleles in release 3.66.0. It carries first-field resolution only.