Establishing race-, gender- and age-specific reference intervals for pyridoxal 5'-phosphate in the NHANES population to better identify adult hypophosphatasia

Establishing reference intervals for thiamine pyrophosphate and pyridoxal 5'-phosphate in whole blood in a Danish cohort using liquid chromatography tandem-mass spectrometry (LC-ms/ms)

Vitamin B1 (thiamine pyrophosphate (TPP)) and B6 (pyridoxal 5'- phosphate (PLP)) deficiencies pose significant health risks. The current measurement method employs High-Performance Liquid Chromatography (HPLC), though, Liquid Chromatography with tandem Mass Spectrometry (LC-MS/MS) is considered a more sensitive and selective analytical method. However, there is a lack of LC-MS/MS-based reference intervals. Moreover, none of the existing reference intervals are established in Danish populations. Therefore, the aim of this study was to establish a reference interval for whole blood concentrations of TPP and PLP in Danish blood donors using LC-MS/MS. Blood samples were collected from healthy Danish blood donors and analysed using the reagent kit, MassChrom® Vitamins B1 and B6 in whole blood (Chromsystems Instruments & Chemicals GmbH, Munich, Germany) for quantitative determination of both TPP and PLP concentration in whole blood, using LC-MS/MS. Reference intervals were determined with non-parametric methods as the 2.5th and 97.5th percentile and presented with 90% confidence intervals (CI). In total 120 blood donors were included. The concentrations of TTP or PLP were not statistically different between sexes just as age did not affect the concentrations, hence, combined reference intervals were employed. The resulting reference intervals are: TPP, nmol/L: 101.0 (90% CI: 96.4-108.5) - 189.0 (90% CI: 184.7-192.0) and PLP, nmol/L: 64.0 (90% CI: 60.9-66.7) - 211.8 (90% CI: 168.3-231.0). In conclusion, reference intervals for whole blood TTP and PLP in a healthy Danish population were established based on a LC-MS/MS method. Furthermore, the reference intervals were not affected by age or sex.

Unsupervised machine learning method for indirect estimation of reference intervals for chronic kidney disease in the Puerto Rican population

Reference intervals (RIs) for clinical laboratory values are extremely important for diagnostics and treatment of patients. However, the determination of these ranges is costly and time-consuming. As a result, often different unverified RIs are used in practice for the same analyte and the same range is used for all patients despite evidence that the values are gender, age, and ethnicity dependent. Moreover, the abnormal flags are rudimentary, merely indicating if a value is within the RI. At the same time, clinical lab data generated in the everyday medical practice contains a wealth of information, that given the correct methodology, can help determine the RIs for each specific segment of the population, including populations that suffer from health disparities. In this work, we develop unsupervised machine learning methods, based on Gaussian mixtures, to determine RIs of analytes related to chronic kidney disease, using millions of routine lab results for the Puerto Rican population. We show that the measures are both gender and age dependent and we find evidence for normal age-related organ function deterioration and failure. We also show that the joint distribution of measures improves the diagnostic value of the lab results.

Current status and challenges in establishing reference intervals based on real-world data

Reference intervals (RIs) are the cornerstone for evaluation of test results in clinical practice and are invaluable in judging patient health and making clinical decisions. Establishing RIs based on clinical laboratory data is a branch of real-world data mining research. Compared to the traditional direct method, this indirect approach is highly practical, widely applicable, and low-cost. Improving the accuracy of RIs requires not only the collection of sufficient data and the use of correct statistical methods, but also proper stratification of heterogeneous subpopulations. This includes the establishment of age-specific RIs and taking into account other characteristics of reference individuals. Although there are many studies on establishing RIs by indirect methods, it is still very difficult for laboratories to select appropriate statistical methods due to the lack of formal guidelines. This review describes the application of real-world data and an approach for establishing indirect reference intervals (iRIs). We summarize the processes for establishing iRIs using real-world data and analyze the principle and applicable scope of the indirect method model in detail. Moreover, we compare different methods for constructing growth curves to establish age-specific RIs, in hopes of providing laboratories with a reference for establishing specific iRIs and giving new insight into clinical laboratory RI research. (201 words).

Bone Turnover Markers: Basic Biology to Clinical Applications

Bone turnover markers (BTMs) are used widely, in both research and clinical practice. In the last 20 years, much experience has been gained in measurement and interpretation of these markers, which include commonly used bone formation markers bone alkaline phosphatase, osteocalcin, and procollagen I N-propeptide; and commonly used resorption markers serum C-telopeptides of type I collagen, urinary N-telopeptides of type I collagen and tartrate resistant acid phosphatase type 5b. BTMs are usually measured by enzyme-linked immunosorbent assay or automated immunoassay. Sources contributing to BTM variability include uncontrollable components (e.g., age, gender, ethnicity) and controllable components, particularly relating to collection conditions (e.g., fasting/feeding state, and timing relative to circadian rhythms, menstrual cycling, and exercise). Pregnancy, season, drugs, and recent fracture(s) can also affect BTMs. BTMs correlate with other methods of assessing bone turnover, such as bone biopsies and radiotracer kinetics; and can usefully contribute to diagnosis and management of several diseases such as osteoporosis, osteomalacia, Paget's disease, fibrous dysplasia, hypophosphatasia, primary hyperparathyroidism, and chronic kidney disease-mineral bone disorder.

Personalized reference intervals: from theory to practice

Using laboratory test results for diagnosis and monitoring requires a reliable reference to which the results can be compared. Currently, most reference data is derived from the population, and patients in this context are considered members of a population group rather than individuals. However, such reference data has limitations when used as the reference for an individual. A patient's test results preferably should be compared with their own, individualized reference intervals (RI), i.e. a personalized RI (prRI).The prRI is based on the homeostatic model and can be calculated using an individual's previous test results obtained in a steady-state situation and estimates of analytical (CV_A) and biological variation (BV). BV used to calculate the prRI can be obtained from the population (within-subject biological variation, CV_I) or an individual's own data (within-person biological variation, CV_P). Statistically, the prediction interval provides a useful tool to calculate the interval (i.e. prRI) for future observation based on previous measurements. With the development of information technology, the data of millions of patients is stored and processed in medical laboratories, allowing the implementation of personalized laboratory medicine. PrRI for each individual should be made available as part of the laboratory information system and should be continually updated as new test results become available.In this review, we summarize the limitations of population-based RI for the diagnosis and monitoring of disease, provide an outline of the prRI concept and different approaches to its determination, including statistical considerations for deriving prRI, and discuss aspects which must be further investigated prior to implementation of prRI in clinical practice.

Vitamin B-6 and riboflavin, their metabolic interaction, and relationship with MTHFR genotype in adults aged 18-102 years

BACKGROUND

The generation of the active form of vitamin B-6, pyridoxal 5'-phosphate (PLP), in tissues is dependent upon riboflavin as flavin mononucleotide, but whether this interaction is important for maintaining vitamin B-6 status is unclear.

OBJECTIVE

To investigate vitamin B-6 and riboflavin status, their metabolic interaction, and relationship with methylenetetrahydrofolate reductase (MTHFR) genotype in adulthood.

METHODS

Data from 5612 adults aged 18-102 y were drawn from the Irish National Adult Nutrition Survey (NANS; population-based sample) and the Trinity-Ulster Department of Agriculture (TUDA) and Genovit cohorts (volunteer samples). Plasma PLP and erythrocyte glutathione reductase activation coefficient (EGRac), as a functional indicator of riboflavin, were determined.

RESULTS

Older (≥65 y) compared with younger (<65 y) adults had significantly lower PLP concentrations (P < 0.001). A stepwise decrease in plasma PLP was observed across riboflavin categories, from optimal (EGRac ≤1.26), to suboptimal (EGRac: 1.27-1.39), to deficient (EGRac ≥1.40) status, an effect most pronounced in older adults (mean ± SEM: 76.4 ± 0.9 vs 65.0 ± 1.1 vs 55.4 ± 1.2 nmol/L; P < 0.001). In individuals with the variant MTHFR 677TT genotype combined with riboflavin deficiency, compared with non-TT (CC/CT) genotype participants with sufficient riboflavin, we observed PLP concentrations of 52.1 ± 2.9 compared with 76.8 ±0.7 nmol/L (P < 0.001). In participants with available dietary data (i.e., NANS cohort, n = 936), PLP was associated with vitamin B-6 intake (nonstandardized regression coefficient β: 2.49; 95% CI 1.75, 3.24; P < 0.001), supplement use (β: 81.72; 95% CI: 66.01, 97.43; P < 0.001), fortified food (β: 12.49; 95% CI: 2.08, 22.91; P = 0.019), and EGRac (β: -65.81; 95% CI: -99.08, -32.54; P < 0.001), along with BMI (β: -1.81; 95% CI: -3.31, -0.30; P = 0.019).

CONCLUSIONS

These results are consistent with the known metabolic dependency of PLP on flavin mononucleotide (FMN) and suggest that riboflavin may be the limiting nutrient for maintaining vitamin B-6 status, particularly in individuals with the MTHFR 677TT genotype. Randomized trials are necessary to investigate the PLP response to riboflavin intervention within the dietary range. The TUDA study and the NANS are registered at www.ClinicalTrials.gov as NCT02664584 (27 January 2016) and NCT03374748 (15 December 2017), respectively.Clinical Trial Registry details: Trinity-Ulster-Department of Agriculture (TUDA) study, ClinicalTrials.gov no. NCT02664584 (January 27th 2016); National Adult Nutrition Survey (NANS), ClinicalTrials.gov no. NCT03374748 (December 15th 2017).

Associations of Serum Vitamin B6 Status and Catabolism With All-Cause Mortality in Patients With T2DM

CONTEXT

There is little evidence regarding the association between serum vitamin B6 status and catabolism and all-cause mortality in patients with type-2 diabetes mellitus (T2DM).

OBJECTIVE

We aimed to ascertain if the serum level of vitamin B6 and catabolism, including pyridoxal 5'-phosphate (PLP) and 4-pyridoxic acid (4-PA), were associated with risk of all-cause mortality in T2DM patients.

METHODS

This prospective cohort study involved 2574 patients with T2DM who participated in the National Health and Nutrition Examination Survey (NHANES) from 2005 to 2010. The serum concentrations of PLP and 4-PA were used to assess the serum level of vitamin B6. Mortality status was determined by routine follow-up using the National Death Index through December 31, 2015.

RESULTS

Over a median follow-up of 85 months, there were 588 deaths. The fully adjusted Cox model indicated that the highest serum PLP concentrations (> 63.6 nmol/L) were associated with a decrease in all-cause mortality (hazard ratio [HR], 0.74; 95% CI, 0.55-0.99, P trend = .035). The risk for all-cause mortality was 59% higher for participants with the highest quartile of 4-PA level compared with the lowest quartile (HR, 1.62; 95% CI, 1.12-2.35; P trend = .003). The sensitivity and specificity of the combination of PLP and 4-PA levels for the prediction of all-cause mortality were 59.5% and 60.9%, respectively (area under the receiver operating characteristic curve = 0.632). The Kaplan-Meier method was used to estimate overall survival for patients based on different combinations of PLP level and 4-PA level. Patients with PLP less than 24.3 nmol/L and 4-PA greater than or equal to 25.4 nmol/L had the worst outcomes (log-rank P < .001).

CONCLUSION

Overall, our data suggest that a low serum level of PLP and high serum level of 4-PA, which represent the serum level of vitamin B6, increases the risk of all-cause mortality significantly in patients with T2DM.

Hypophosphatasia: Vitamin B6 status of affected children and adults

Hypophosphatasia (HPP) is the heritable dento-osseous disease caused by loss-of-function mutation(s) of the gene ALPL that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). TNSALP is a cell-surface homodimeric phosphomonoester phosphohydrolase expressed in healthy people especially in the skeleton, liver, kidneys, and developing teeth. In HPP, diminished TNSALP activity leads to extracellular accumulation of its natural substrates including inorganic pyrophosphate (PPi), an inhibitor of mineralization, and pyridoxal 5'-phosphate (PLP), the principal circulating form of vitamin B₆ (B₆). Autosomal dominant and autosomal recessive inheritance involving >450 usually missense defects scattered throughout ALPL largely explains the remarkably broad-ranging severity of this inborn-error-of-metabolism. In 1985 when we identified elevated plasma PLP as a biochemical hallmark of HPP, all 14 investigated affected children and adults had markedly increased PLP levels. However, pyridoxal (PL), the dephosphorylated form of PLP that enters cells to cofactor many enzymatic reactions, was not low but often inexplicably elevated. Levels of pyridoxic acid (PA), the B₆ degradation product quantified to assess B₆ sufficiency, were unremarkable. Canonical signs or symptoms of B₆ deficiency or toxicity were absent. B₆-dependent seizures in infants with life-threatening HPP were later explained by their profound deficiency of TNSALP activity blocking PLP dephosphorylation to PL and diminishing gamma-aminobutyric acid synthesis in the brain. Now, there is speculation that altered B₆ metabolism causes further clinical complications in HPP. Herein, we assessed the plasma PL and PA levels accompanying previously reported elevated plasma PLP concentrations in 150 children and adolescents with HPP. Their mean (SD) plasma PL level was nearly double the mean for our healthy pediatric controls: 66.7 (59.0) nM versus 37.1 (22.2) nM (P < 0.0001), respectively. Their PA levels were broader than our pediatric control range, but their mean value was normal; 40.2 (25.1) nM versus 39.3 (9.9) nM (P = 0.7793), respectively. In contrast, adults with HPP often had plasma PL and PA levels suggestive of dietary B₆ insufficiency. We discuss why the B₆ levels of our pediatric patients with HPP would not cause B₆ toxicity or deficiency, whereas in affected adults dietary B₆ insufficiency can develop.

Hypophosphatasia

Hypophosphatasia (HPP) is an inherited metabolic disease caused by loss-of-function mutations in the tissue non-specific alkaline phosphatase (TNAP) gene. Reduced activity of TNAP leads to the accumulation of its substrates, mainly inorganic pyrophosphate and pyridoxal-5′-phosphate, metabolic aberrations that largely explain the musculoskeletal and systemic features of the disease. More than 400 ALPL mutations, mostly missense, are reported to date, transmitted by either autosomal dominant or recessive mode. Severe disease is rare, with incidence ranging from 1:100,000 to 1:300,000 live births, while the estimated prevalence of the less severe adult form is estimated to be between 1:3100 to 1:508, in different countries in Europe. Presentation largely varies, ranging from death in utero to asymptomatic adults. In infants and children, clinical features include skeletal, respiratory and neurologic complications, while recurrent, poorly healing fractures, muscle weakness and arthropathy are common in adults. Persistently low serum alkaline phosphatase is the cardinal biochemical feature of the disease. Management requires a dedicated multidisciplinary team. In mild cases, treatment is usually symptomatic. Severe cases, with life-threating or debilitating complications, can be successfully treated with enzyme replacement therapy with asfotase alfa.

Biochemical, clinical and genetic characteristics in adults with persistent hypophosphatasaemia; Data from an endocrinological outpatient clinic in Denmark

Background

Hypophosphatasia (HPP) is an inborn disease caused by pathogenic variants in ALPL. Low levels of alkaline phosphatase (ALP) are a biochemical hallmark of the disease. Scarce knowledge about the prevalence of HPP in Scandinavia exists, and the variable clinical presentations make diagnostics challenging. The aim of this study was to investigate the prevalence of ALPL variants as well as the clinical and biochemical features among adults with endocrinological diagnoses and persistent hypophosphatasaemia.

Methods

A biochemical database containing ALP measurements of 26,121 individuals was reviewed to identify adults above 18 years of age with persistently low levels of ALP beneath range (≤ 35 ± 2.7 U/L). ALPL genetic testing, biochemical evaluations and assessment of clinical features by a systematic questionnaire among included patients, were performed.

Results

Among 24 participants, thirteen subjects (54.2%) revealed a disease-causing variant in ALPL and reported mild clinical features of HPP, of which musculoskeletal pain was the most frequently reported (n = 9). The variant c. 571G > A; p.(Glu191Lys) was identified in six subjects, and an unreported missense variant (c.1019A > C; p.(His340Pro)) as well as a deletion of exon 2 were detected by genetic screening. Biochemical analyses showed no significant differences in ALP (p = 0.059), the bone specific alkaline phosphatase (BALP) (p = 0.056) and pyridoxal-5′-phosphate (PLP) (p = 0.085) between patients with an ALPL variant and negative genetic screening. Patients with a variant in ALPL had significantly higher PLP levels than healthy controls (p = 0.002). We observed normal ALP activity in some patients classified as mild HPP, and slightly increased levels of PLP in two subjects with normal genetic screening and four healthy controls. Among 51 patients with persistent hypophosphatasaemia, fifteen subjects (29.4%) received antiresorptive treatment. Two patients with unrecognized HPP were treated with bisphosphonates and did not show complications due to the treatment.

Conclusions

Pathogenic variants in ALPL are common among patients with endocrinological diagnoses and low ALP. Regarding diagnostics, genetic testing is necessary to identify mild HPP due to fluctuating biochemical findings. Antiresorptive treatment is a frequent reason for hypophosphatasaemia and effects of these agents in adults with a variant in ALPL and osteoporosis remain unclear and require further studies.

•

Mild HPP is overlooked among adults with endocrinological diagnoses.

•

Biochemical hallmarks of HPP can fluctuate to normal among mild forms.

•

Genetic screening of ALPL including MLPA is necessary to identify mild HPP.

•

Musculoskeletal pain is the most common symptom in adult HPP.

•

Effects of bisphosphonates in mild HPP need further studies.

Clinical and biochemical characteristics of adults with hypophosphatasia attending a metabolic bone clinic

OBJECTIVES

This study sought to identify the clinical and biochemical characteristics that would help distinguish hypophosphatasia (HPP) from other metabolic bone diseases in adult patients attending a metabolic bone clinic by comparing patients who have genetically confirmed HPP with a group of patients with low bone mineral density (BMD) in the osteoporotic or osteopenic range.

METHODS

Data were collected from February 2016 to October 2018 for 41 patients (n = 20 in the HPP group, n = 21 in the low-BMD group) attending the metabolic bone clinic at Sheffield, United Kingdom (UK) or who were recruited via the Rare UK Diseases Study (RUDY) platform during the same period. A study questionnaire was administered to all patients, and assessments were conducted for laboratory values, physical functions, BMD, and spine imaging.

RESULTS

Patients with HPP were characterized as being younger, more likely to have metatarsal or femoral shaft fractures, and less likely to have vertebral fractures compared with patients in the low-BMD group. The HPP group had lower total and bone-specific alkaline phosphatase, higher pyridoxal 5'-phosphate (PLP), and lower, albeit sufficient, 25-hydroxyvitamin D. Low-BMD group had lower C-terminal telopeptide and tartrate-resistant acid phosphatase 5b (61.9% were on bisphosphonates at enrollment). Dual X-ray absorptiometry (DXA) analysis found that the HPP group had higher total hip and lumbar BMD T- and Z-scores compared with the low-BMD group. There were no differences found between the two groups with physical functional assessments. Results of receiver operating characteristic analysis indicated strong diagnostic accuracy of these biomarkers for HPP. Thresholds of total alkaline phosphatase (ALP) activity of 43 IU/L or less and PLP level of 120 nmol/L or more were determined to be potentially clinically useful for distinguishing HPP from other metabolic bone diseases.

CONCLUSION

This study supported the use of ALP and PLP measurements as predictive of HPP diagnosis along with certain demographic and clinical characteristics (younger age, metatarsal or femoral fractures without low mean BMD T- and Z-scores on a DXA scan) that can aid in recognizing adults who should be further evaluated for HPP. The critical values identified need to be applied to an independent sample to be tested for diagnostic accuracy.