Globulin and A:G Ratio Calculator
Globulin and A:G Ratio Calculator
Calculate globulin as total protein minus albumin, with the A:G ratio and what a reversed ratio actually tells you about the two components.
Globulin and A:G Ratio
Alb ÷ (TP − Alb)Total protein 7.2 g/dL, albumin 4.1 g/dL
Formula
A:G ratio = albumin ÷ globulin
- total protein
- measured by the biuret reaction — the sum of albumin and all globulins
- globulin
- never measured directly; a derived value that carries the analytical error of both the total protein and the albumin assay
- same sample
- both values must come from one sample in one unit — subtracting across draws, laboratories or units gives a meaningless globulin
Worked example
Total protein 7.2 g/dL, albumin 4.1 g/dL
Globulin = 7.2 − 4.1 = 3.1 g/dL
4.1 ÷ 3.1 = 1.32
Between 1.0 and 2.5 → within the normal range
Reading the ratio
| A:G ratio | Pattern | Consider |
|---|---|---|
| < 1.0 | Reversed | Chronic liver disease, myeloma or another paraprotein, chronic infection, nephrotic syndrome |
| 1.0 – 2.5 | Normal range | No protein abnormality suggested by the ratio alone |
| > 2.5 | High | Low globulin — hypogammaglobulinaemia, primary or secondary antibody deficiency |
Which component produced the ratio
| Pattern | Albumin | Globulin | Typical causes |
|---|---|---|---|
| Reversed by a low albumin | Low | Normal | Nephrotic syndrome, protein-losing enteropathy, acute illness, liver failure |
| Reversed by a high globulin | Normal | High | Myeloma or another paraprotein, chronic infection, autoimmune hepatitis |
| Reversed by both | Low | High | Chronic liver disease, advanced myeloma with renal involvement |
| High ratio | Normal | Low | Hypogammaglobulinaemia, primary antibody deficiency |
A derived value, and why the components matter more
Globulin is not measured. It is calculated as total protein minus albumin, which means it inherits the analytical error of both assays and has no reference method of its own. Total protein is measured by the biuret reaction and albumin usually by a dye-binding method, and because bromocresol green reads albumin higher than bromocresol purple does, the same sample yields a lower calculated globulin — and a higher A:G ratio — on a green assay than on a purple one. A value built by subtracting two independent measurements is inherently noisier than either of them.
The pattern worth recognising is the reversed ratio, where globulin exceeds albumin and the ratio falls below one. It is the classic finding in chronic liver disease, in myeloma and other paraproteinaemias, and in chronic infection and inflammation. But the ratio alone does not say which of its two components produced it, and the two work-ups have nothing in common. A reversed ratio driven by a low albumin points towards liver failure, nephrotic syndrome, protein-losing enteropathy or acute illness; one driven by a high globulin points towards a plasma cell or inflammatory process. Always read the components rather than the ratio.
A raised globulin fraction should prompt serum protein electrophoresis. That is the test that separates a broad-based polyclonal rise, typical of chronic infection, autoimmune disease or chronic liver disease, from a discrete monoclonal band — a paraprotein — which then requires immunofixation, serum free light chains and, depending on the result, further haematological assessment. The size of the globulin figure can suggest which is more likely, but it cannot make the distinction.
A low globulin is easier to overlook and can matter as much. It suggests hypogammaglobulinaemia and should prompt measurement of immunoglobulins, since primary antibody deficiency in an adult is often diagnosed years after the recurrent infections began. Finally, both inputs must come from the same sample: subtracting an albumin from one draw from a total protein from another, or mixing g/dL with g/L, produces a globulin that means nothing at all.
Frequently asked questions
How is globulin calculated?
Globulin is total protein minus albumin, both measured on the same sample and in the same units. It is never measured directly, so it carries the analytical error of both assays.
What does a reversed A:G ratio mean?
It means globulin exceeds albumin, giving a ratio below one. This is the classic pattern in chronic liver disease, myeloma and chronic infection, but the ratio alone does not say whether a low albumin or a high globulin produced it.
What should I do about a raised globulin fraction?
Request serum protein electrophoresis. It separates a broad-based polyclonal rise, seen in chronic infection, autoimmune disease and chronic liver disease, from a discrete monoclonal band, which needs immunofixation and serum free light chains.
Does the albumin assay affect the result?
Yes. Bromocresol green reads albumin higher than bromocresol purple, so the same sample gives a smaller calculated globulin and a higher A:G ratio on a green method. Serial values are only comparable within one laboratory.
What does a low globulin suggest?
Hypogammaglobulinaemia, and it should prompt measurement of immunoglobulins. Primary antibody deficiency in adults is frequently diagnosed years after recurrent infections begin, so a low globulin is worth pursuing rather than dismissing.
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References
- Busher JT. Serum albumin and globulin. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Butterworths; 1990.
- Bird J, Behrens J, Westin J, et al. UK Myeloma Forum and Nordic Myeloma Study Group guidelines for the investigation of newly detected M-proteins. Br J Haematol. 2009;147(1):22–42.
- Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018.