Controversies Around the Measurement of Blood Ketones to Diagnose and Manage Diabetic Ketoacidosis Free

Blood ketone measurement has become an established test in supporting the diagnosis and management of diabetic ketoacidosis (DKA) and has largely supplanted the use of urine ketones in clinical practice (1). Its measurement is used in other clinical situations, including the investigation of some inborn errors of metabolism and assessing the response to a ketogenic diet, but these indications are not discussed in this article.

Despite the test being generally widely adopted, there remain several outstanding questions associated with the test’s use, including the following:

1. Should blood ketone measurement be used as a central tenet for the initial diagnosis of DKA, and should it help guide further treatment?

2. What is the preferred test to measure blood ketones? Should it use the nitroprusside reaction or specifically measure β-hydroxybutyrate (BOHB)?

3. What is the evidence to support the recommended diagnostic and management blood ketone concentration thresholds?

4. Should measurement be performed in the laboratory or as a point-of-care test (POCT)?

This article has reviewed the existing literature, including current major guidelines, to help answer some of these questions and to highlight the deficiencies (and therefore research opportunities) where a paucity of evidence exists.

Ketoacidosis refers to the metabolic acidosis resulting from the overproduction of acetoacetate and BOHB anions and their accompanying equimolar amounts of hydrogen ions (2). It therefore might be assumed that the diagnosis and severity of DKA would be best served, or at least complemented, by the quantitative measurement of blood ketones rather than the consequential effect on pH or HCO₃. Indeed, other acid-base disturbances, such as lactic acidosis, or compensations to the initial ketone-induced acidosis could be superimposed on the pH or HCO₃ to over- or underestimate the degree to which ketosis contributes to the overall clinical picture.

Although the 2009 American Diabetes Association (ADA) consensus guidelines require the presence of urine or blood ketones to make a biochemical diagnosis of DKA, the document focuses predominantly on a patient’s hyperglycemia (glucose >250 mg/L [13.9 mmol/L]), arterial pH, serum bicarbonate concentration, and anion gap (3). The presence of DKA is judged by the pH, bicarbonate, and anion gap, with reference to blood ketones simply being that they should be positive in all cases, as defined by measurement specifically using the nitroprusside reaction. Some limitations of the nitroprusside method are described in the guidelines (described below), which probably helps explain the reason they subsequently state that blood BOHB measurement “if available…may be useful for diagnosis.” A more recent consensus document from eight professional organizations, including the ADA, the American Association of Clinical Endocrinologists (AACE), the Endocrine Society, the Pediatric Endocrine Society, and JDRF International, describes no distinction or preference between using blood or urine ketones for DKA diagnosis (4).

In contrast, 2011 laboratory guidelines from the ADA and the American Association for Clinical Chemistry wholeheartedly endorse the measurement of BOHB in blood to diagnose and monitor the course of DKA while specifically recommending against using urine ketones for this purpose (2). They recommend reserving blood ketones that rely on the nitroprusside reaction only as an adjunct to diagnose DKA but for it to have no role in monitoring DKA treatment. U.K. Joint British Diabetes Societies (JBDS) guidelines, originally from the same year (5), also favor using blood BOHB over urine ketone testing for reasons including being able to more readily identify euglycemic DKA, which is more common in pregnancy, patients with partially treated DKA, and now in patients prescribed sodium–glucose cotransporter 2 (SGLT2) inhibitors. Perhaps because of the increasing prevalence of SGLT2 inhibitor use, euglycemic DKA features more prominently in this latest version of this guidance (6). International Society for Pediatric and Adolescent Diabetes (ISPAD) guidelines from 2018 recommend measuring blood BOHB in preference to urine ketones to establish a DKA diagnosis “whenever possible” (7), reflecting that such testing is not universally available, and pediatric guidance from the National Institute for Health and Care Excellence from 2021 describes a similar preference (8).

Once a DKA diagnosis has been made, judging its severity is important, as it can inform the most appropriate intensity of clinical setting for a patient to be treated. The role of blood ketone concentrations in helping make this initial severity assessment remains controversial. As summarized previously (9,10), traditional factors associated with severity, such as pH, bicarbonate concentrations, or blood glucose levels, correlate with blood BOHB concentrations more poorly than might be anticipated, which has meant its use for this purpose is seldom recommended. Only JBDS guidelines make mention of this scenario by stating it can be one of the nine clinical or biochemical criteria influencing high-dependency unit admission, immediate senior review, and/or insertion of a central line (5). Even so, this is only when unequivocally high BOHB concentrations (>6 mmol/L; to convert to mg/dL, multiply by approximately 10) are present.

Despite this general view that blood ketones are of little utility in this clinical situation, the authors of this current article question whether a lack of correlation with traditional severity markers automatically means it is inferior to them, especially as the opposite assumption has been made with respect to DKA diagnosis (see evidence for diagnostic and management thresholds), where the ketone result has been presented as more of the gold standard.

There is some disparity regarding opinions in using blood ketones to assess the success, or otherwise, of subsequent DKA management. ADA guidelines recommend that continuous intravenous insulin be administered until “ketonemia is controlled” but do not elaborate on how this should be assessed (3). ISPAD endorses the use of blood BOHB measurement for both diagnosis and response to treatment, suggesting, where available, that it is measured every 2 h, aiming for a BOHB reduction rate of approximately 0.5 mmol/L/h until the concentration is <1 mmol/L (7). The JBDS guidelines also recommend that repeated measurement of BOHB be central to achieving metabolic targets with the aim to reduce concentrations by at least 0.5 mmol/L/h (5).

• There is inconsistency in the recommended use of blood ketones for either the diagnosis or management of DKA. Some of this disparity likely relates to the publication date of guidelines as blood ketone technology has developed.

• For hospital care, the most contemporary guidance documents recommend including blood ketone, particularly BOHB, measurement in helping to determine the diagnosis, for treatment monitoring, and to indicate resolution of DKA.

• It remains uncertain whether there is an additional role for using blood ketone measurement to assess the initial severity of a DKA episode. The current article authors see there being a research opportunity to clarify this further.

Ketone bodies comprise BOHB (not strictly a ketone) as well as acetoacetate and acetone, the former being the major component in DKA and the latter being the most minor. Blood ketones are routinely measured either quantitatively as BOHB or semiquantitatively (+, ++, etc.) using the same nitroprusside reaction as that commonly employed in urine ketone testing (2). Measurement of blood ketones by laboratories in the U.S. is regulated and the quality of testing assured by the College of American Pathologists. Recent College of American Pathologists records show there is a split in what is measured, such that, in 2020 (KET-04), 1,785 laboratories were measuring specifically BOHB while 840 were measuring a reaction using nitroprusside. In contrast, the equivalent quality schemes in the U.K. show that no laboratories measure blood ketones using the nitroprusside test.

There are reasons why BOHB measurement could be considered superior to nitroprusside beyond it being a more quantitative measurement. The nitroprusside test principally measures acetoacetate, not BOHB, and since BOHB predominates in ketoacidosis, the degree of ketonemia using nitroprusside initially could be underestimated (2). In addition, when the acidosis has resolved, BOHB is more readily oxidized to acetoacetate, so overall ketosis paradoxically may appear to be worsening when the converse is true (9). Blood BOHB measurement is also available as both a laboratory test and a POCT, whereas no routinely used POCTs for blood nitroprusside testing exists, thereby potentially limiting its use (described below).

• Within the U.S. at least, there is a laboratory split when measuring blood ketones between the use of BOHB and the nitroprusside method, with the latter method predominantly measuring acetoacetate.

• Given the likely clinical advantages to measuring BOHB as well as the option for its measurement as a POCT, BOHB measurement should be the preferred blood ketone test.

Table 1 gives a summary of recommended thresholds from several guidelines when using BOHB to help support a diagnosis of ketoacidosis. The evolution of the 3-mmol/L cutoff includes the recommendation stated as a “preliminary guideline,” which suggested this level based on 14 patients admitted with DKA who were diagnosed by other contemporaneous means (11,12). This was reinforced by a highly referenced study of 129 children and 337 adults that recommended a BOHB cutoff of 3.0 and 3.8 mmol/L for children (<16 years) and adults, respectively (13). These thresholds were derived from the blood BOHB concentrations, which were, on average, equivalent to a bicarbonate concentration of 18 mmol/L, so there is something of a circular argument, or at least a collinearity, with the way existing DKA diagnostic criteria were established. Where there was a disparity in that study between HCO₃ and BOHB concentrations (the R² for all ages was between 0.64 and 0.68), it was suggested by the authors that BOHB would be the more reliable guide, although they admitted that this “must be arrived at on theoretical grounds” (13). The subsequent widespread decision to adopt a 3-mmol/L threshold for all age groups based on this work presumably reflected taking a conservative approach. Most guidelines that do not directly refer to these articles refer to documents that either do or do not provide a source for their cutoffs.

A notable guideline exception concerning a diagnostic blood BOHB threshold is the consensus report in 2017 from 8 U.S. professional organizations, including the AACE, the ADA, and the Endocrine Society (4). It recommends that “serum ketones” greater than “the upper limit of the normal range” be part of the DKA definition. If expressed as blood BOHB, then this could equate to concentrations of just 0.6 mmol/L or greater, which has led some to question whether this could lead to overdiagnosis of DKA (14).

Other factors beyond patient age are thought to affect the relationship between bicarbonate and BOHB concentration, an example being end-stage kidney disease. One study showed that in DKA patients with bicarbonate concentrations <18 mmol/L, the mean BOHB concentration in those with end-stage kidney disease was 1.4 mmol/L lower than in patients with preserved renal function (15).

The source of intermediate thresholds, such as 0.6–1.5 mmol/L, suggesting ketonemia, or 1.6–3 mmol/L, suggesting impending DKA (16), seems more difficult to establish with clarity. The preliminary guideline from Wallace et al. suggested that BOHB concentrations <1 mmol/L were not associated with incipient ketosis, whereas concentrations between 1 and 3 mmol/L required further intervention (11). A number of subsequent studies, however, refer to the BOHB concentrations suggested by the ketone meter manufacturers themselves (17–19). In turn, the manufacturer thresholds stated in the Abbott/Medisense instructions seem largely based on a study where an elevated BOHB was defined as being >0.5 mmol/L and where a value >1.5 mmol/L was one of their diagnostic criteria for DKA (20). However, this study did not reference the source of these cutoffs; in fact, >1.5 mmol/L was their diagnostic threshold rather than potentially being an intermediate level. The “upper limit of normal” concentration may date back to a more historic study of fasting patients; this is not clear (21), although this study did identify some normal values, up to 1 mmol/L, in some individuals, as has been noted by others (22). More recently, nonfasting samples have been used to establish a much tighter reference interval of 0.02 mmol/L to 0.28 mmol/L, although the confidence intervals around these limits meant the upper reference value could truly be as high as 0.5 mmol/L; no sex difference was apparent (23).

Despite the variance in cutoffs and the uncertainty in their provenance, there has been some clinical verification of the 1.5-mmol/L value as a useful intermediate threshold: a systematic review (24) identified that in 2 studies a BHOB concentration >1.5 mmol/L, compared with standard measures of DKA, was associated with a positive predictive value of 50% and a negative predictive value of 100% (19,25).

When monitoring the response to treatment of DKA, the JBDS reached their conclusion of aiming for BOHB to fall by at least 0.5 mmol/L/h through guideline group consensus and based upon previous work, which suggested “BOHB could be expected to fall by 1 mmol/L per hour” (5,11). ISPAD guidance of approximately 0.5 mmol/L/h cited a study of children and seems to have extrapolated its recommendation from the data found there (7,26).

The recommended levels of blood BOHB used for the identification and further management of euglycemic DKA are identical to that of hyperglycemic DKA (4,6). Indeed, one guideline has dispensed with plasma glucose as a criterion for DKA diagnosis in known diabetes patients, largely because patients can present with euglycemia (4). There is some evidence that resolution from the episode takes longer than anticipated when SGLT2 inhibitors have contributed to its development (27).

• The blood BOHB threshold most commonly quoted to define DKA (3 mmol/L) seems mainly based on its equivalence to a serum bicarbonate of 18 mmol/L. The relationship between these two parameters may vary between patient groups.

• Intermediate BOHB thresholds, such as 1.5 mmol/L, for lesser degrees of ketosis have in some studies been used as a diagnostic threshold instead. Nonetheless, there is some evidence that it is a helpful level in excluding DKA.

• There are arguments for using either 0.6 or 1 mmol/L BOHB as the trigger for action/inaction.

• Despite these points, it seems prudent to continue to use the 3-mmol/L diagnostic threshold, but there is value in further studies verifying or disproving this as well as the normal threshold.

Blood ketone BOHB measurement can be performed as a laboratory test or using handheld meters as a POCT. Blood testing using the nitroprusside reaction is, as mentioned above, limited to the laboratory setting. The convenience of testing and rapidity of results from a POCT instrument in a hospital setting are quoted as main reasons for implementing blood ketone measurement in both ISPAD and U.K. JBDS guidelines (6,7). Certainly, a systematic review has shown benefits of POCT blood ketone measurement concerning reducing time for assessment, duration of admission, and time to recovery from DKA (1), although it is not possible to know if rapid laboratory measurement would also have translated into a similar clinical benefit for patients.

Despite the appeal of ketone meters, some concern has been voiced about how reliable POCT instruments, even contemporary ones, are for measuring blood BOHB concentrations, although the issue is usually at BOHB concentrations above 5 mmol/L, which is well in excess of any DKA diagnostic threshold (28,29). This issue might not only impact the identification of hyperketonemia but also, together with meter result imprecision, cause unreliable tracking in the rate of fall of BOHB concentrations (30). The latter point has led to a recommendation that blood ketone measurement trends not be relied upon in isolation from other factors, such as change in pH, and for users to be aware of the limitations of the test, as for any POCT (31). This last point also leads to other considerations that need to be taken into account before implementing any hospital POCT, including the requirement to train and competency assess all staff on how to use the meter, have a quality control program in place, and whether the result will be as consistently recorded in the electronic patient record as would normally be the case with a laboratory analysis (31).

Discussing the reliability of meter measurement raises the question of what analytical performance should be regarded as clinically acceptable. There is an opportunity to clinically define how good this should be for optimal patient care (the so-called analytical performance specification [32]), which, if beyond current meter analytical capability, might help drive the development of instruments that can achieve this degree of performance.

In the U.S., it is not possible to easily assess the current proportion of blood ketone measurements performed as POCT versus laboratory tests, since blood ketone meters are regarded by the Food and Drug Administration as “waived” tests that do not require the same quality checks as a laboratory test. In the U.K., where these POCT checks are more commonplace, there is an obvious preference for BOHB meters, with over 1,650 POC instrument users registered compared with just 12 laboratories registered for the same test. A survey in the U.K. in 2014 showed 76% of institutions could measure blood BOHB by POCT (33).

• Although being widely implemented in at least some health care systems, it is not known if outcomes from studies employing point-of-care blood ketone testing would have been different if rapid laboratory measurement was used instead.

• While convenient and rapid, point-of-care blood ketone measurement may have analytical limitations compared with laboratory measurement; therefore, careful planning is required prior to their implementation.

• Recommendations for acceptable performance specifications for POCT blood BOHB instruments would help ensure meters provide clinically acceptable results.

The contemporary identification and management of patients with suspected DKA makes use of the measurement of blood ketones more than some guidelines currently recommend. There seems to be good evidence for specifically measuring BOHB rather than acetoacetate derived from the nitroprusside reaction for at least the diagnosis of DKA, although if or how it might be used during further management of the condition is less clear. Point-of-care blood BOHB measurement has advantages over laboratory measurement, although safeguards with regard to staff training and instrument performance need to be in place. There are research opportunities, such as verifying the currently used diagnostic blood BOHB thresholds, to more clearly define the role of blood ketones in the DKA pathway.

Duality of Interest. No potential conflicts of interest relevant to this article were reported.