Bone alkaline phosphatase isoenzyme in renal osteodystrophy

Bone alkaline phosphatase in CKD-mineral bone disorder

Overall and cardiovascular mortality in patients with chronic kidney disease (CKD) is greatly increased, without obvious current effective treatments. Mineral and bone disorder (MBD) is a common manifestation of CKD and contributes to the high risk of fracture and cardiovascular mortality in these patients. Traditionally, clinical management of CKD-MBD focused on attenuation of secondary hyperparathyroidism due to impaired renal activation of vitamin D and phosphate retention, although recently, adynamic forms of renal bone disease have become more prevalent. Definitive diagnosis was based on histologic (histomorphometric) analysis of bone biopsy material supported by radiologic changes and changes in levels of surrogate laboratory markers. Of these various markers, parathyroid hormone (PTH) has been considered to be the most sensitive and currently is the most frequently used; however, the many pitfalls of measuring PTH in patients with CKD increasingly are appreciated. We propose an alternative or complementary approach using bone alkaline phosphatase (ALP), which is directly related to bone turnover, reflects bone histomorphometry, and predicts outcomes in hemodialysis patients. Here, we consider the overall merits of bone ALP as a marker of bone turnover in adults with CKD-MBD, examine published bone histomorphometric data comparing bone ALP to PTH, and discuss possible pathogenic mechanisms by which bone ALP may be linked to outcomes in patients with CKD.

Bone alkaline phosphatase besides intact parathyroid hormone in hemodialysis patients--any advantage?

BACKGROUND/AIM

Bone alkaline phosphatase (bAP) is known to be an important biochemical marker of bone formation. Through the present study, we intended to find out whether there is any advantage in bAP determination, as a routine biochemical marker, besides intact parathyroid hormone (iPTH) in hemodialysis patients.

METHODS

In a population of 140 hemodialysis patients, bAP and iPTH were determined on four quarterly consecutive occasions. According to the values of iPTH (pg/ml) and bAP (ng/ml), patients were divided into four groups: group I: iPTH > 200 and bAP > 20, group II: iPTH > 200 and bAP < 20, group III: iPTH < 200 and bAP < 20 and group IV: iPTH < 200 and bAP > 20. Patients with higher serum phosphorus (P) (group A: P > or = 7 mg/dl) were compared with those with lower serum P levels (group B: P < 7 mg/dl).

RESULTS

The global correlation between iPTH and bAP (total evaluations, n = 503) was 0.32 (p < 0.001). Group IV patients tended to show a slight increase of serum aluminum (sAl) levels, which were 12.48 +/- 5.35 microg/l higher than in the patients from group I (sAl = 9.97 +/- 4.39 microg/l), group II (sAl = 10.86 +/- 4.45 microg/l) or group III (sAl = 10.92 +/- 3.92 microg/l). Significance values (Mann-Whitney) in each group, in comparison with group IV, were the following: group I: 0.004; group II: 0.062; group III: < 0.001. Group A (n = 66) showed higher iPTH levels than group B (n = 430), although bAP and sAl were both similar in these two groups of patients (Mann-Whitney): iPTH (A) = 631.0 +/- 487.7 vs. iPTH (B) = 253.3 +/- 191.6, p < 0.001; bAP (A) = 22.9 +/- 17.4 vs. bAP (B) = 20.4 +/- 13.1, p = n.s.; sAl (A) = 10.2 +/- 3.5 vs. sAl (B) = 10.8 +/- 4.4, p = n.s. For similar Al and bAP values, group A showed a much stronger iPTH/bAP correlation than group B: r = 0.67 (p < 0.001) vs. r = 0.30 (p < 0.001), respectively.

CONCLUSION

Although iPTH and bAP are frequently in agreement, it seems important to separate parathyroid activity given by iPTH, from bone remodelling reflected by bAP, in the presence of either a higher aluminum exposition or a well-controlled phosphatemia.

Biochemical Indices of Renal Osteodystrophy in Dialysis Patients on the Island of Malta

OBJECTIVES

Renal osteodystrophy (ROD) has never been studied on the small Mediterranean island of Malta, which has a largely inbred population. The genetic contribution to the pattern of renal osteodystrophy is being increasingly recognized. We were, thus, interested in studying indices of bone turnover in Maltese end stage renal failure patients.

MATERIALS AND METHODS

Sixty unselected patients, representing 65% of all patients undergoing dialysis in the island's renal unit, were prospectively investigated over a period of 5 months with respect to symptoms, calcium/phosphate chemistry, intact parathyroid hormone (iPTH) and bone alkaline phosphatase (bAP). Bone histomorphometry, which is the gold standard in the diagnosis of ROD, was not within the reach of our small unit. Biochemical markers may not be as sensitive and specific as bone biopsy for individual patient diagnosis of ROD sub-type but they can give a fairly good indication of the spectrum of bone turnover on a population basis. The optimum combination of biochemical marker cut-offs available from studies in the literature was then employed to estimate bone turnover.

RESULTS

The following biochemical picture emerged: 42% had iPTH < 79.7 pg/ml (which cut off has a reported specificity of 93.7% for low turnover bone disease), 45% had iPTH > 100 pg/ml and bAP > 10 ng/ml (which combined cut off has a reported specificity of 100% for high turnover bone disease), while 13% could not be classified (ie had intermediate values).

CONCLUSIONS

Based on biochemical data, the pattern of bone turnover seems to be comparable to the European average. Further indepth study using bone histomorphometry is warranted.

Bone disease in patients with chronic kidney disease under conservative management

CONTEXT AND OBJECTIVE

Few studies have focused on bone disease in patients with chronic kidney disease under conservative treatment. The objective was to evaluate bone disease in patients with chronic kidney disease.

DESIGN AND SETTING

Case series, at the Nephrology Division, Hospital Universitário Pedro Ernesto.

METHODS

131 patients with creatinine clearance from 10 to 60 ml/min/1.73 m(2) were followed up for at least one year. Serum creatinine, albumin, calcium, phosphorus, alkaline phosphatase, total CO2 (tCO2), intact parathyroid hormone (iPTH), and alkaline phosphatase were measured. Creatinine clearance was calculated from 24-hour urine creatinine measurements and protein ingestion estimates from urea assays.

RESULTS

Patients presenting creatinine clearance < 30 ml/min/1.73 m(2) had higher iPTH values, but normal serum levels for calcium, phosphorus, alkaline phosphatase and tCO2. Patients presenting iPTH values of twice the normal upper limit (144 pg/ml) showed lower tCO2 values. Bone alkaline phosphatase was evaluated in 37 patients with creatinine clearance < 30 ml/min/1.73 m(2), showing correlation with alkaline phosphatase but not with parathyroid hormone. Bone biopsy on nine patients with creatinine clearance < 30 ml/min/1.73 m(2) and iPTH > 144 pg/ml showed osteitis fibrosa (4), mild lesion (4) and high turnover (1).

CONCLUSION

The present data suggest the importance of early control for iPTH and metabolic acidosis, among patients under conservative management for chronic kidney disease, in order to prevent complications related to bone disease.

Diagnosis and monitoring of renal osteodystrophy

A variety of biochemical investigations and radiological techniques are available to assist in the diagnosis and monitoring of renal osteodystrophy. Measurement of serum parathyroid hormone remains the single most useful biochemical test in predicting bone histology in an individual patient. Newer biochemical markers of bone turnover are unlikely to supplant this in everyday practice, but may provide useful supplementary information in the future. The present review discusses the role of radiological investigation, including bone densitometry and quantitative ultrasound. Bone biopsy remains the 'gold standard' investigation. Its invasive nature and the need for specialized processing and interpretation limits its use in clinical practice, although it still has a role particularly in the investigation of low turnover states. Also, as molecular biological techniques are increasingly being used, the evaluation of biopsy specimens will in the future provide new insights into the disordered bone cell function that occurs in renal osteodystrophy.

Biological markers in the diagnosis of the different forms of renal osteodystrophy

Renal osteodystrophy continues to be a long-term complication associated with high rates of morbidity in patients with chronic renal failure. Although bone histomorphometry is the most reliable diagnostic method, several new biochemical markers of bone turnover have been proposed in recent years for the evaluation of bone remodelling in uremic patients. This review assesses the value and the limitations of serum markers of bone formation and resorption in the diagnosis of the major types of renal osteodystrophy. In addition, we consider the hypothetical role of serum beta2-microglobulin and of some local mediators involved in the process of bone cell activation and inhibition, such as circulating cytokines and their inhibitors and receptors.

A new assay method that detects only intact osteocalcin. Two-step non-invasive diagnosis to predict adynamic bone disease in haemodialysed patients

BACKGROUND

We studied the usefulness of a new assay method that detects only the intact human osteocalcin molecule in haemodialysed patients.

METHODS

Iliac bone biopsy specimens obtained from 62 haemodialysed patients were analysed.

RESULTS

Bone formation rates (BFR/BS) correlated positively with serum intact osteocalcin concentrations (n=62), osteocalcin concentrations assayed by a conventional method (n=31), parathyroid hormone (PTH) concentrations (n=62), and total alkaline phosphatase concentrations (r=0.602, 0. 588, 0.650, and 0.401 respectively). Based on ROC curve and Youden index analysis, the optimal cut-off value to distinguish adynamic bone disease from a mild lesion was 195 pg/ml of serum PTH concentration (Youden index=0.233) or 30 ng/ml of serum intact osteocalcin concentration (Youden index=0.628). The optimal cut-off value to distinguish between hyperparathyroid bone and a mild lesion was 455 pg/ml of serum PTH level (Youden index=0.63) or 50 ng=ml of serum intact osteocalcin concentration (Youden index=0.634). Since both ROC curve and Youden index suggested that the serum PTH concentration was not a good marker to distinguish adynamic bone from a mild lesion or hyperparathyroid bone, we designed a two-step procedure. The first step was the diagnosis of adynamic bone (cut-off: 65 pg/ml) or hyperparathyroid bone (cut-off: 455 pg/ml) according to serum PTH concentration. In a second step, we assessed serum intact osteocalcin concentration in patients with serum PTH concentrations between 65 and 455 pg/ml. The cut-off values for adynamic and hyperparathyroid bone in this diagnostic approach were 30 and 70 ng/ml respectively. As a result, 49 out of 62 patients were diagnosed properly. The Youden index of this two-step diagnosis was 0.527 and 0.661 for adynamic bone and hyperparathyroid bone respectively. Sensitivity markedly improved to 94.4% and 96.2% for adynamic bone and hyperparathyroid bone respectively, without sacrificing specificity (84.0 and 88.8% respectively).

CONCLUSION

Measurement of serum intact osteocalcin concentration is useful for the diagnosis of adynamic bone in haemodialysed patients. A two-step procedure involving also simultaneous measurement of serum PTH concentration further improved the sensitivity of each individual marker while maintaining specificity.

Use of Bone Alkaline Phosphatase to Monitor Alendronate Therapy in Individual Postmenopausal Osteoporotic Women

Background: Biochemical bone markers are sensitive to the changes in bone turnover that result from treatment of postmenopausal osteoporotic women with antiresorptive therapies. Although information is available on the use of bone markers in monitoring therapy in groups of subjects, less is known regarding how these markers perform in individual patients.

Methods: Serum bone alkaline phosphatase (bone ALP) concentrations, measured with the Tandem® Ostase® assay, were used to monitor the biochemical response of bone in postmenopausal women with osteoporosis receiving either 10 mg/day alendronate therapy (n = 74) or calcium supplementation (n = 148) for 24 months.

Results: Bone ALP decreased significantly from baseline at 3 months (P ≤0.0001), reaching a nadir between 3 and 6 months of alendronate therapy. The magnitude of the bone ALP decrease in the treated osteoporotic population was consistent with normalization to premenopausal concentrations. Of the 74 alendronate-treated subjects, 63 (85.1%) demonstrated a decrease from baseline in bone ALP by 6 months that exceeded the least significant change of 25%. The bone ALP decrease from baseline exceeded 25% in 72 (97%) by the end of the study.

Conclusion: The bone ALP assay is a sensitive and reliable tool that may be used to monitor the reduction in bone turnover after alendronate therapy in individual postmenopausal osteoporotic women.

Circulating biochemical markers of bone remodeling in uremic patients

Chronic renal failure is often associated with bone disorders, including secondary hyperparathyroidism, aluminum-related low-turnover bone disease, osteomalacia, adynamic osteopathy, osteoporosis, and skeletal beta2-microglobulin amyloid deposits. In spite of the enormous progress made during the last few years in the search of noninvasive methods to assess bone metabolism, the distinction between high- and low-turnover bone diseases in these patients still frequently requires invasive and/or costly procedures such as bone biopsy after double tetracycline labeling, scintigraphic-scan studies, computed tomography, and densitometry. This review is focused on the diagnostic value of several new serum markers of bone metabolism, including bone-specific alkaline phosphatase (bAP), procollagen type I carboxy-terminal extension peptide (PICP), procollagen type I cross-linked carboxy-terminal telopeptide (ICTP), pyridinoline (PYD), osteocalcin, and tartrate-resistant acid phosphatase (TRAP) in patients with chronic renal failure. Most of the observations made by several groups converge to the conclusion that serum bAP is the most sensitive and specific marker to evaluate the degree of bone remodeling in uremic patients. Nonetheless, PYD and osteocalcin, in spite of their retention and accumulation in the serum of renal insufficient patients, are also excellent markers of bone turnover. The future generalized use of these markers, individually or in combination with other methods, will undoubtedly improve the diagnosis and the treatment of the complex renal osteodystrophy.