A1C and 1,5-anhydroglucitol do not compete for the same job. A1C is an established test for diabetes screening, diagnosis, and longer-term monitoring. 1,5-anhydroglucitol (1,5-AG) is a narrower adjunctive marker that may fall when glucose has recently exceeded the kidney’s reabsorption threshold and spilled into urine.
In selected people, a low 1,5-AG result may add context when A1C appears acceptable or unchanged but recent high-glucose excursions—often after meals—are still suspected. It is not an accepted stand-alone test for diagnosing diabetes, and it becomes unreliable for glycemic monitoring in people taking an SGLT2 inhibitor.
For the full diagnostic framework, see Diabetes and Prediabetes Blood Tests and Prediabetes and Type 2 Diabetes: Why A1C, Fasting Glucose, OGTT, Kidney, and Lipid Testing Must Be Read Together.
The original article compared A1C with 1,5-AG as though a reader needed to choose one winner. That framing created two problems:
The better question is:
When might 1,5-AG provide information about recent glucose excursions that A1C does not show?
The answer depends on the clinical question, medications, kidney function, and whether more direct glucose data are available.
Hemoglobin A1C measures the percentage of hemoglobin with glucose attached. Because red blood cells circulate for weeks to months, A1C estimates longer-term average glucose exposure, with greater influence from more recent weeks.
A1C is widely used because it is standardized, convenient, and linked to established diagnostic criteria. It does not require fasting. However, an average cannot show every high and low. Two people can have the same A1C but very different day-to-day patterns.
A1C may be less representative when red-cell turnover or hemoglobin biology changes. Important examples include iron or B12 deficiency, hemolysis, recent blood loss or transfusion, kidney failure, pregnancy, erythropoietin treatment, and some hemoglobin variants.[2]
When A1C does not match direct glucose data, the discrepancy should be investigated rather than “corrected” by assuming either test is automatically right.
1,5-AG is a naturally occurring monosaccharide found in blood. It is filtered by the kidneys and usually reabsorbed in the renal tubules. When plasma glucose becomes high enough to produce glycosuria, glucose interferes with 1,5-AG reabsorption. More 1,5-AG is lost in urine, and its blood concentration falls.[3]
This mechanism creates an inverse relationship:
The marker responds over a shorter interval than A1C—often described as approximately several days to two weeks. It is especially sensitive to glucose excursions that cross the renal threshold, but the exact threshold varies among people and circumstances.
1,5-AG does not identify exactly when a spike occurred, how high it rose, how long it lasted, whether hypoglycemia also occurred, or which meal or medication caused the pattern.
| Feature | A1C | 1,5-AG |
|---|---|---|
| Primary role | Established screening, diagnosis, and longer-term monitoring | Selected adjunct for recent hyperglycemic excursions |
| Approximate window | About two to three months | Recent days to roughly two weeks |
| Direction with hyperglycemia | Usually rises | Usually falls |
| Main biological basis | Glucose attachment to hemoglobin | Renal loss when glycosuria interferes with tubular reabsorption |
| Shows exact spikes? | No | No |
| Used to diagnose diabetes? | Yes, with standardized methods and confirmation rules | No—do not use as a stand-alone diagnostic test |
| Major limitations | Red-cell turnover, hemoglobin variants, kidney failure, pregnancy, recent transfusion, and other factors | SGLT2 inhibitors, altered renal glucose handling, kidney/tubular disease, pregnancy, variable thresholds and reference intervals |
| Best interpreted with | Plasma glucose, clinical context, and glucose monitoring when needed | A1C, plasma glucose, medication list, kidney function, and direct glucose data |
1,5-AG may be considered when all of the following are true:
Examples include an A1C that changes little despite high post-meal finger-stick readings, or a clinical concern that recent medication or nutrition changes produced short-term highs not yet reflected fully in A1C.
Even in these situations, 1,5-AG is an adjunct. It should not independently trigger medication changes.
Use standardized A1C, fasting plasma glucose, or two-hour 75-g OGTT criteria. 1,5-AG is not a substitute.
SGLT2 inhibitors lower glucose partly by increasing urinary glucose excretion. That mechanism lowers 1,5-AG independently of the glycemic question the test is supposed to answer. Current FDA labeling states that 1,5-AG measurements are unreliable for assessing glycemic control in people taking SGLT2 inhibitors and recommends alternative methods.[1]
Common medications in this class include agents containing empagliflozin, dapagliflozin, canagliflozin, ertugliflozin, and bexagliflozin. Medication lists and combination products should be reviewed carefully.
Because the test depends on filtration and tubular reabsorption, altered kidney physiology can change 1,5-AG independently of recent glucose excursions. Interpretation should include creatinine/eGFR, albuminuria, and the clinical kidney history when relevant.
Pregnancy changes renal glucose handling and uses specialized diabetes screening and monitoring protocols. 1,5-AG should not replace established pregnancy-specific testing or obstetric care.
When glucose is repeatedly high and A1C is already markedly elevated, the key problem is not hidden excursions. 1,5-AG may be very low and provide little additional actionable discrimination.
A low 1,5-AG reflects recent high-glucose exposure through its renal mechanism; it does not measure low glucose. Use direct glucose monitoring for hypoglycemia.
Continuous glucose monitoring (CGM) and structured home glucose testing can show the timing, peak, duration, and frequency of glucose changes. They can also reveal hypoglycemia, which 1,5-AG cannot.
Laboratory markers still have value:
But none of these laboratory markers recreates a glucose trace. When the clinical question is “What happens after breakfast?” a timed glucose measurement or CGM is often more direct than an indirect biomarker.
Use the laboratory’s assay-specific reference interval. Do not apply a universal cutoff copied from a study population or another assay.
A practical interpretation sequence is:
| Pattern | Possible meaning | What to check next |
|---|---|---|
| A1C near goal + low 1,5-AG | Recent substantial glucose excursions may be occurring despite an acceptable average. | Review CGM or timed post-meal glucose, medications, kidney function, and diet/activity context. |
| A1C high + low 1,5-AG | Persistent hyperglycemia is already evident; 1,5-AG may add little. | Prioritize direct glucose assessment and clinical management. |
| A1C high + relatively preserved 1,5-AG | Results may reflect different windows, an A1C influence, or assay/renal factors. | Confirm A1C, plasma glucose, CBC/iron/B12 or hemoglobin context when indicated. |
| A1C near goal + preserved 1,5-AG | No strong 1,5-AG signal of recent glycosuria-producing excursions. | This does not exclude smaller spikes, lows, or variability below the renal threshold. |
| Any A1C + low 1,5-AG while taking an SGLT2 inhibitor | Expected drug-related interference is likely. | Do not use 1,5-AG to assess glycemic control; use alternative monitoring. |
| Test | Best use in this question | Key limitation |
|---|---|---|
| 1,5-Anhydroglucitol (1,5-AG) | Selected adjunct for recent glycosuria-producing hyperglycemic excursions | Not diagnostic; unreliable with SGLT2 inhibitors and affected by renal physiology. |
| Hemoglobin A1C | Established longer-term glycemic exposure and diagnostic criterion | Can hide variability and be distorted by red-cell factors. |
| Glucose | Direct glucose concentration at collection | One moment; preparation and timing matter. |
| Two-hour 75-g OGTT | Standardized detection of post-challenge dysglycemia | Longer visit; does not provide day-to-day monitoring. |
| Comprehensive Metabolic Panel | Glucose, creatinine/eGFR, electrolytes, and related context | Does not replace UACR or direct glucose pattern data. |
| Urine Albumin/Creatinine Ratio | Kidney-damage context in diabetes | Temporary elevations occur; not a measure of glucose excursions. |
No. A1C has an established role in diagnosis and long-term monitoring. 1,5-AG has a narrower adjunctive role for selected recent hyperglycemia questions.
It may indicate recent glucose high enough to increase urinary glucose, but it cannot prove that the excursions occurred after meals or show their timing. Direct glucose data are needed for that level of detail.
No. CGM shows glucose over time, including highs, lows, timing, and duration. 1,5-AG is an indirect summary influenced by renal glucose handling.
It can respond over days as urinary glucose exposure changes, with common descriptions covering approximately several days to two weeks. The rate varies and should not be used as a precise clock.
Not to monitor glycemic control. FDA prescribing information states that measurements are unreliable in this setting and recommends alternative methods.
No. Smaller spikes below the renal threshold, hypoglycemia, and rapid fluctuations may not lower 1,5-AG meaningfully.
Ulta Lab Tests provides direct access to many laboratory tests where available. Before ordering, review the Complete Guide to Lab Tests and Blood Work, How to Read and Understand Your Lab Results, and Direct-Access Lab Testing: A Complete Guide.
Testing should answer a defined question. A specialized marker is not automatically more informative than an established test or direct glucose data.
A1C and 1,5-AG measure different biological signals. A1C is the established longer-term marker. 1,5-AG may add short-term context when recent glycosuria-producing glucose excursions are suspected and A1C does not reveal them. Its value is limited by medication and kidney physiology—especially SGLT2 inhibitor use—and it should never replace standard diagnostic criteria, CGM, home glucose data, or clinical judgment.
Originally published: October 9, 2023 | Substantively updated: August 27 2026
Editorial disclaimer: This article is educational and does not establish a diagnosis or treatment plan. Medication changes and glucose-management decisions require qualified clinical guidance
Primary glycemic markers
Kidney/renal-glucose context
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