Correlation between urine anion gap and urine ammonia-creatinine ratio in healthy cats and cats with kidney disease

BACKGROUND

Ammonium excretion decreases as kidney function decreases in several species, including cats, and may have predictive or prognostic value in patients with chronic kidney disease (CKD). Urine ammonia measurement is not readily available in clinical practice, and urine anion gap (UAG) has been proposed as a surrogate test.

OBJECTIVES

Evaluate the correlation between urine ammonia-to-creatinine ratio (UACR) and UAG in healthy cats and those with CKD and determine if a significant difference exists between UAG of healthy cats and cats with CKD.

ANIMALS

Urine samples collected from healthy client-owned cats (n = 59) and those with stable CKD (n = 17).

METHODS

Urine electrolyte concentrations were measured using a commercial chemistry analyzer and UAG was calculated as ([sodium] + [potassium]) - [chloride]. Urine ammonia and creatinine concentrations had been measured previously using commercially available enzymatic assays and used to calculate UACR. Spearman's rank correlation coefficient between UAG and UACR was calculated for both groups. The UAG values of healthy cats and cats with CKD were assessed using the Mann-Whitney test (P < .05).

RESULTS

The UAG was inversely correlated with UACR in healthy cats (P < .002, r0  = -0.40) but not in cats with CKD (P = .55; r0  = -0.15). A significant difference was found between UAG in healthy cats and those with CKD (P < .001).

CONCLUSIONS AND CLINICAL IMPORTANCE

The UAG calculation cannot be used as a substitute for UACR in cats. The clinical relevance of UAG differences between healthy cats and those with CKD remains unknown.

Urine anion gap can differentiate respiratory alkalosis from metabolic acidosis in the absence of blood gas results

INTRODUCTION

Low plasma bicarbonate concentration due to chronic respiratory alkalosis may be misdiagnosed as metabolic acidosis and mistreated with administration of alkali therapy, particularly when arterial blood gas is not available.

METHODS

We measured urine anion gap [urine (Na⁺  + K⁺ ) - (Cl^- )], as a surrogate of renal ammonium excretion in 15 patients presenting with hyperventilation and low serum bicarbonate concentration to distinguish chronic respiratory alkalosis (CRA) from metabolic acidosis (MA) when blood gas was unavailable.

RESULTS

Hyperventilation and low serum bicarbonate concentrations were associated with urine pH above 5.5 and positive urine anion gap in all, suggesting CRA. The diagnosis was later confirmed by obtaining capillary blood gas, which showed a decrease in PCO₂ and high normal pH values.

CONCLUSION

The use of urine anion gap can help to differentiate between chronic respiratory alkalosis and metabolic acidosis, especially when arterial blood gas is not obtained.

Hypokalemia-Induced Cardiac Arrest

Renal tubular acidosis (RTA) refers to a group of disorders in which the elimination of hydrogen ions from the kidney or the reabsorption of filtered bicarbonate is impaired, resulting in metabolic acidosis. Hypokalemia is also prominent in different types of RTA. We are presenting an interesting case about a chronic alcoholic patient who presented to the emergency department and was found to be severely hypokalemic. During her hospital stay, she had multiple cardiac arrests likely secondary to her hypokalemia despite adequate treatment with potassium supplementation. We came to the conclusion of distal RTA in our patient based on hyperchloremic metabolic acidosis, sodium bicarbonate of 10 mmol/L, low potassium, blood urea nitrogen, and creatinine within normal limits, alkaline urine, and a positive urinary anion gap. It is likely that the cause of our patient's underlying type 1 RTA was secondary to her chronic alcohol abuse. Her potassium eventually returned to baseline, and she was discharged.

Assessing urine ammonium concentration by urine osmolal gap in chronic kidney disease

Acidemia is one of the risk factors for end-stage kidney disease and increases the mortality rate of patients with chronic kidney disease (CKD). Although urinary ammonium (U-NH₄ ⁺ ) is the crucial component of renal acid excretion, U-NH₄ ⁺ concentration is not routinely measured. To estimate U-NH₄ ⁺ , urine osmolal gap (UOG = urine osmolality - [2(Na⁺ + K⁺ ) + urea + glucose]) is calculated and the formula (U-NH₄ ⁺  = UOG/2) has traditionally been used. However, the usefulness of this formula is controversial in CKD patients. We assessed the relationship between U-NH₄ ⁺ and UOG in patients with CKD. Blood and spot urine samples were collected in 36 patients who had non-dialysis-dependent CKD. The mean ± SD age of patients was 72.0 ± 14.8 years, and the mean ± SD serum creatinine and U-NH₄ ⁺ were 2.7 ± 2.3 mg/dl and 9.3 ± 9.2 mmol/L, respectively. A significant relationship was found between UOG/2 and U-NH₄ ⁺ (r = .925, p < .0001). U-NH₄ ⁺ estimated using the UOG was on average higher by 4.7 mmol/L than the measured one. Our results suggested that UOG could be a useful tool in clinical settings, especially in patients with moderate to severe CKD.

The role of the clinical laboratory in diagnosing acid-base disorders

Acid-base homeostasis is fundamental for life. The body is exceptionally sensitive to changes in pH, and as a result, potent mechanisms exist to regulate the body's acid-base balance to maintain it in a very narrow range. Accurate and timely interpretation of an acid-base disorder can be lifesaving but establishing a correct diagnosis may be challenging. The underlying cause of the acid-base disorder is generally responsible for a patient's signs and symptoms, but laboratory results and their integration into the clinical picture is crucial. Important acid-base parameters are often available within minutes in the acute hospital care setting, and with basic knowledge it should be easy to establish the diagnosis with a stepwise approach. Unfortunately, many caveats exist, beginning in the pre-analytical phase. In the post-analytical phase, studies on the arterial reference pH are scarce and therefore many different reference values are used in the literature without any solid evidence. The prediction models that are currently used to assess the acid-base status are approximations that are mostly based on older studies with several limitations. The two most commonly used methods are the physiological method and the base excess method, both easy to use. The secondary response equations in the base excess method are the most convenient. Evaluation of acid-base disorders should always include the assessment of electrolytes and the anion gap. A major limitation of the current acid-base laboratory tests available is the lack of rapid point-of-care laboratory tests to diagnose intoxications with toxic alcohols. These intoxications can be fatal if not recognized and treated within minutes to hours. The surrogate use of the osmolal gap is often an inadequate substitute in this respect. This article reviews the role of the clinical laboratory to evaluate acid-base disorders.

Urine Anion Gap to Predict Urine Ammonium and Related Outcomes in Kidney Disease

BACKGROUND AND OBJECTIVES

Low urine ammonium excretion is associated with ESRD in CKD. Few laboratories measure urine ammonium, limiting clinical application. We determined correlations between urine ammonium, the standard urine anion gap, and a modified urine anion gap that includes sulfate and phosphate and compared risks of ESRD or death between these ammonium estimates and directly measured ammonium.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We measured ammonium, sodium, potassium, chloride, phosphate, and sulfate from baseline 24-hour urine collections in 1044 African-American Study of Kidney Disease and Hypertension participants. We evaluated the cross-sectional correlations between urine ammonium, the standard urine anion gap (sodium + potassium - chloride), and a modified urine anion gap that includes urine phosphate and sulfate in the calculation. Multivariable-adjusted Cox models determined the associations of the standard urine anion gap and the modified urine anion gap with the composite end point of death or ESRD; these results were compared with results using urine ammonium as the predictor of interest.

RESULTS

The standard urine anion gap had a weak and direct correlation with urine ammonium (r=0.18), whereas the modified urine anion gap had a modest inverse relationship with urine ammonium (r=-0.58). Compared with the highest tertile of urine ammonium, those in the lowest urine ammonium tertile had higher risk of ESRD or death (hazard ratio, 1.46; 95% confidence interval, 1.13 to 1.87) after adjusting for demographics, GFR, proteinuria, and other confounders. In comparison, participants in the corresponding standard urine anion gap tertile did not have higher risk of ESRD or death (hazard ratio, 0.82; 95% confidence interval, 0.64 to 1.07), whereas the risk for those in the corresponding modified urine anion gap tertile (hazard ratio, 1.32; 95% confidence interval, 1.03 to 1.68) approximated that of directly measured urine ammonium.

CONCLUSIONS

Urine anion gap is a poor surrogate of urine ammonium in CKD unless phosphate and sulfate are included in the calculation. Because the modified urine anion gap merely estimates urine ammonium and requires five measurements, direct measurements of urine ammonium are preferable in CKD.

Non-Anion Gap Metabolic Acidosis: A Clinical Approach to Evaluation

Acid-base disturbances can result from kidney or nonkidney disorders. We present a case of high-volume ileostomy output causing large bicarbonate losses and resulting in a non-anion gap metabolic acidosis. Non-anion gap metabolic acidosis can present as a form of either acute or chronic metabolic acidosis. A complete clinical history and physical examination are critical initial steps to begin the evaluation process, followed by measuring serum electrolytes with a focus on potassium level, blood gas, urine pH, and either direct or indirect urine ammonium concentration. The present case was selected to highlight the differential diagnosis of a non-anion gap metabolic acidosis and illustrate a systematic approach to this problem.

Olmesartan-induced Enteropathy Manifesting as Wernicke-Korsakoff Syndrome

Cases of sprue-like enteropathy associated with olmesartan have sporadically been encountered since it was first reported in 2012, and their most characteristic manifestation is severe diarrhea. We herein report the first case of sprue-like enteropathy manifesting as Wernicke-Korsakoff syndrome due to vitamin B1 malabsorption with only minimally increased bowel movements. When patients are receiving olmesartan and they complain of nonspecific chronic gastrointestinal symptoms, it is important to consider changing the drugs before any serious malabsorption syndrome develops.

Distal renal tubular acidosis associated with Sjogren syndrome

Renal tubular acidosis is a common cause of normal anion gap metabolic acidosis but these disorders can be easily missed or misdiagnosed. We highlight the approach to assessing renal tubular acidosis by discussing a case study with a temporal data set collected over more than 5 weeks. We highlight the principles and the necessary information required for a diagnosis of classic distal renal tubular acidosis. We also briefly review several aspects of type 1 renal tubular acidosis related to autoimmune disease, drugs and thyroid disorders.