Disorders of plasma phosphate and indications for its measurement

Synthesis of Cu (II) and Zn (II) Complexes of a Quinoline Based Flexible Amide Receptor as Fluorescent Probe for Dihydrogen Phosphate and Hydrogen Sulphate and Their Antibacterial Activity

A novel 8-hydroxy quinoline-derived amide receptor, in conjunction with its Cu (II) and Zn (II) complexes, has been strategically developed to function as remarkably efficient fluorescent receptors with a distinct capability for anion sensing. The comprehensive characterization of the synthesized compounds were achieved through UV-Vis, IR, NMR, and HRMS spectroscopic techniques. Among the Cu (II) and Zn (II) complexes, the latter exhibits superior selectivity for anions, specifically dihydrogen phosphate and hydrogen sulfate, as their tetrabutylammonium salts in a 9:1 acetonitrile-water (v/v) mixture. The Cu (II) complex demonstrates enhanced anion binding compared to the amide ligand, albeit with reduced selectivity. Furthermore, the affinity was evaluated using the Benesi-Hildebrand plot. The binding constants and Limit of Detection (LOD) for both complexes were precisely quantified. The Job plot illustrates a clear 1:1 binding interaction between the metal complexes and the guest anions. Significantly, both metal-complex receptors display a broad spectrum of antibacterial activity, against both gram-positive and gram-negative bacteria. It is worth highlighting that the Zn (II) complexed receptor outperforms the Cu (II) complexed receptor, as evidenced by its considerably lower Minimum Inhibitory Concentration (MIC) value against both bacterial strains.

Candida albicans' inorganic phosphate transport and evolutionary adaptation to phosphate scarcity

Phosphorus is essential in all cells' structural, metabolic and regulatory functions. For fungal cells that import inorganic phosphate (Pi) up a steep concentration gradient, surface Pi transporters are critical capacitators of growth. Fungi must deploy Pi transporters that enable optimal Pi uptake in pH and Pi concentration ranges prevalent in their environments. Single, triple and quadruple mutants were used to characterize the four Pi transporters we identified for the human fungal pathogen Candida albicans, which must adapt to alkaline conditions during invasion of the host bloodstream and deep organs. A high-affinity Pi transporter, Pho84, was most efficient across the widest pH range while another, Pho89, showed high-affinity characteristics only within one pH unit of neutral. Two low-affinity Pi transporters, Pho87 and Fgr2, were active only in acidic conditions. Only Pho84 among the Pi transporters was clearly required in previously identified Pi-related functions including Target of Rapamycin Complex 1 signaling and hyphal growth. We used in vitro evolution and whole genome sequencing as an unbiased forward genetic approach to probe adaptation to prolonged Pi scarcity of two quadruple mutant lineages lacking all 4 Pi transporters. Lineage-specific genomic changes corresponded to divergent success of the two lineages in fitness recovery during Pi limitation. In this process, initial, large-scale genomic alterations like aneuploidies and loss of heterozygosity were eventually lost as populations presumably gained small-scale mutations. Severity of some phenotypes linked to Pi starvation, like cell wall stress hypersensitivity, decreased in parallel to evolving populations' fitness recovery in Pi scarcity, while that of others like membrane stress responses diverged from these fitness phenotypes. C. albicans therefore has diverse options to reconfigure Pi management during prolonged scarcity. Since Pi homeostasis differs substantially between fungi and humans, adaptive processes to Pi deprivation may harbor small-molecule targets that impact fungal growth and virulence.

Glycerophosphocholine provision rescues Candida albicans growth and signaling phenotypes associated with phosphate limitation

IMPORTANCE

Candida albicans is the most commonly isolated species from patients suffering from invasive fungal disease. C. albicans is most commonly a commensal organism colonizing a variety of niches in the human host. The fungus must compete for resources with the host flora to acquire essential nutrients such as phosphate. Phosphate acquisition and homeostasis have been shown to play a key role in C. albicans virulence, with several genes involved in these processes being required for normal virulence and several being upregulated during infection. In addition to inorganic phosphate (Pi), C. albicans can utilize the lipid-derived metabolite glycerophosphocholine (GPC) as a phosphate source. As GPC is available within the human host, we examined the role of GPC in phosphate homeostasis in C. albicans. We find that GPC can substitute for Pi by many though not all criteria and is likely a relevant physiological phosphate source for C. albicans.

The emerging role of phosphorus in human health

Phosphorus, an essential nutrient, performs vital functions in skeletal and non-skeletal tissues and is pivotal for energy production. The last two decades of research on the physiological importance of phosphorus have provided several novel insights about its dynamic nature as a nutrient performing functions as a phosphate ion. Phosphorous also acts as a signaling molecule and induces complex physiological responses. It is recognized that phosphorus homeostasis is critical for health. The intake of phosphorus by the general population world-wide is almost double the amount required to maintain health. This increase is attributed to the incorporation of phosphate containing food additives in processed foods purchased by consumers. Research findings assessed the impact of excessive phosphorus intake on cells' and organs' responses, and highlighted the potential pathogenic consequences. Research also identified a new class of bioactive phosphates composed of polymers of phosphate molecules varying in chain length. These polymers are involved in metabolic responses including hemostasis, brain and bone health, via complex mechanism(s) with positive or negative health effects, depending on their chain length. It is amazing, that phosphorus, a simple element, is capable of exerting multiple and powerful effects. The role of phosphorus and its polymers in the renal and cardiovascular system as well as on brain health appear to be important and promising future research directions.

Importance of Dietary Phosphorus for Bone Metabolism and Healthy Aging

Inorganic phosphate (Pi) plays a critical function in many tissues of the body: for example, as part of the hydroxyapatite in the skeleton and as a substrate for ATP synthesis. Pi is the main source of dietary phosphorus. Reduced bioavailability of Pi or excessive losses in the urine causes rickets and osteomalacia. While critical for health in normal amounts, dietary phosphorus is plentiful in the Western diet and is often added to foods as a preservative. This abundance of phosphorus may reduce longevity due to metabolic changes and tissue calcifications. In this review, we examine how dietary phosphorus is absorbed in the gut, current knowledge about Pi sensing, and endocrine regulation of Pi levels. Moreover, we also examine the roles of Pi in different tissues, the consequences of low and high dietary phosphorus in these tissues, and the implications for healthy aging.

Disorders of phosphate metabolism

Phosphate in both inorganic and organic form is essential for several functions in the body. Plasma phosphate level is maintained by a complex interaction between intestinal absorption, renal tubular reabsorption, and the transcellular movement of phosphate between intracellular fluid and bone storage pools. This homeostasis is regulated by several hormones, principally the parathyroid hormone, 1,25-dihydroxyvitamin D and fibroblast growth factor 23. Abnormalities in phosphate regulation can lead to serious and fatal complications. In this review phosphate homeostasis and the aetiology, pathophysiology, clinical features, investigation and management of hypophosphataemia and hyperphosphataemia will be discussed.

Phosphate as a Signaling Molecule and Its Sensing Mechanism

In mammals, phosphate balance is maintained by influx and efflux via the intestines, kidneys, bone, and soft tissue, which involves multiple sodium/phosphate (Na+/Pi) cotransporters, as well as regulation by several hormones. Alterations in the levels of extracellular phosphate exert effects on both skeletal and extra-skeletal tissues, and accumulating evidence has suggested that phosphate itself evokes signal transduction to regulate gene expression and cell behavior. Several in vitro studies have demonstrated that an elevation in extracellular Piactivates fibroblast growth factor receptor, Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway and Akt pathway, which might involve the type III Na+/Picotransporter PiT-1. Excessive phosphate loading can lead to various harmful effects by accelerating ectopic calcification, enhancing oxidative stress, and dysregulating signal transduction. The responsiveness of mammalian cells to altered extracellular phosphate levels suggests that they may sense and adapt to phosphate availability, although the precise mechanism for phosphate sensing in mammals remains unclear. Unicellular organisms, such as bacteria and yeast, use some types of Pitransporters and other molecules, such as kinases, to sense the environmental Piavailability. Multicellular animals may need to integrate signals from various organs to sense the phosphate levels as a whole organism, similarly to higher plants. Clarification of the phosphate-sensing mechanism in humans may lead to the development of new therapeutic strategies to prevent and treat diseases caused by phosphate imbalance.

Fluorescence Sensing of Inorganic Phosphate and Pyrophosphate Using Small Molecular Sensors and Their Applications

The aim of this contribution is to provide an introduction and a brief summary of the principle of fluorescence molecular sensors specific to inorganic phosphate (Pi) and inorganic pyrophosphate (PPi) as well as their applications. In our introduction we describe the impact of both Pi and PPi in the living organism and in the environment, followed by a description of the principle of fluorescence molecular sensors and the sensing mechanism in solution. We then focus on exciting research which has emerged in recent years on the development of fluorescent sensors specific to Pi and PPi, categorized by chemical interactions between the sensor and the target molecule, such as hydrogen bonding, coordination chemistry, displacement assay, aggregation induced emission or quenching, and chemical reactions.

Value of calcium and phosphate in a bicarbonate-containing platelet additive solution with low plasma levels in maintaining key in vitro platelet storage parameters

BACKGROUND

Use of recently developed platelet (PLT) additive solutions (PAS) with 5% plasma levels may reduce the frequency and/or severity of transfusion reactions attributed to plasma. PLTs suspended in bicarbonate-containing PAS-5 with 5% plasma levels can maintain key PLT parameters during 7-day storage. This study evaluates the role of calcium and phosphate, as constituents of PAS-5, in maintaining PLT parameters.

STUDY DESIGN AND METHODS

An Amicus apheresis PLT unit (n = 13) was equally divided into four 60-mL aliquots in CF-250 polyolefin bags. Four different formulations of PAS-5 were prepared: PAS-5, PAS-5 without phosphate (-PO₄ ), PAS-5 without calcium (-Ca), and PAS-5 without Ca and phosphate (-Ca/-PO₄ ). PLTs were centrifuged, and the supernatant was expressed and replaced with the respective PAS, yielding PLTs suspended in 95% PAS and 5% plasma. PLTs were stored at 20 to 24ºC with agitation for 7 days. PLT in vitro parameters were evaluated on Days 1, 5, and 7.

RESULTS

In PLT PAS-5 aliquots, pH levels were maintained better compared with those in -Ca and -Ca/-PO₄ aliquots. Glycolysis was greater in -Ca and -Ca/-PO₄ PLT aliquots compared with PAS-5 aliquots. Hypotonic stress response and morphology were less and p-selectin (CD62P) binding was greater in -Ca/-PO₄ PLT aliquots. The accumulation of reactive oxygen species was greater in -Ca/-PO₄ PLTs. Phosphorylation of p38 mitogen-activated protein kinase (MAPK) was greater in -Ca and -Ca/-PO₄ PLT aliquots during storage.

CONCLUSION

The removal of calcium and phosphate from PAS-5 leads to the activation of p38 MAPK and deterioration of key PLT storage parameters.

The regulation of renal phosphate transport

Renal phosphate transport is mediated by the abundance and activity of the sodium-dependent phosphate transporters, Npt2a, Npt2c, and PiT-2, present within the apical brush border membrane of the proximal tubule. Recent studies have demonstrated differential expression and activity of these sodium-dependent phosphate transporters within the proximal tubule. In general, phosphate transport is regulated by a variety of physiological stimuli, including parathyroid hormone, glucocorticoids, vitamin D3, estrogen, and thyroid hormone. Phosphatonins are now recognized as major regulators of phosphate transport activity. Other factors that affect phosphate transport include dopamine, dietary phosphate, acid-base status, lipid composition, potassium deficiency, circadian rhythm, and hypertension. Studies have shown that the PDZ-containing sodium/hydrogen exchanger regulatory factor (NHERF) proteins, specifically NHERF-1 and NHERF-3, play a critical role in the physiological regulation of phosphate transport, particularly in response to dietary phosphate. In addition, recent studies have found that NHERF-1 is also important in both the parathyroid hormone- and dopamine-mediated inhibition of phosphate transport. This review will detail the various hormones and agents involved in the regulation of phosphate transport as well as provide a brief summary of the signaling pathways and cytoskeletal proteins active in the transport of phosphate in the renal proximal tubule.

The emergence of phosphate as a specific signaling molecule in bone and other cell types in mammals

Although considerable advances in our understanding of the mechanisms of phosphate homeostasis and skeleton mineralization have recently been made, little is known about the initial events involving the detection of changes in the phosphate serum concentrations and the subsequent downstream regulation cascade. Recent data has strengthened a long-established hypothesis that a phosphate-sensing mechanism may be present in various organs. Such a phosphate sensor would detect changes in serum or local phosphate concentration and would inform the body, the local environment, or the individual cell. This suggests that phosphate in itself could represent a signal regulating multiple factors necessary for diverse biological processes such as bone or vascular calcification. This review summarizes findings supporting the possibility that phosphate represents a signaling molecule, particularly in bone and cartilage, but also in other tissues. The involvement of various signaling pathways (ERK1/2), transcription factors (Fra-1, Runx2) and phosphate transporters (PiT1, PiT2) is discussed.

Identification of a novel function of PiT1 critical for cell proliferation and independent of its phosphate transport activity

PiT1 is a Na(+)-phosphate (P(i)) cotransporter located at the plasma membrane that enables P(i) entry into the cell. Its broad tissue expression pattern has led to the idea that together with the closely related family member PiT2, PiT1 is the ubiquitous supplier of P(i) to the cell. Moreover, the role of P(i) in phosphorylation reactions, ATP production, DNA structure, and synthesis has led to the view that P(i) availability could be an important determinant of cell growth. However, these issues have not been clearly addressed to date, and the role of either P(i) or PiT proteins in cell proliferation is unknown. Using RNA interference in HeLa and HepG2 cells, we show that transient or stable PiT1 depletion markedly reduces cell proliferation, delays cell cycle, and impairs mitosis and cytokinesis. In vivo, PiT1 depletion greatly reduced tumor growth when engineered HeLa cells were injected into nude mice. We provide evidence that this effect on cell proliferation is specific to PiT1 and not shared by PiT2 and is not the consequence of impaired membrane Na(+)-P(i) transport. Moreover, we show that modulation of cell proliferation by PiT1 is independent from its transport function because the proliferation of PiT1-depleted cells can be rescued by non-transporting PiT1 mutants. PiT1 depletion leads to the phosphorylation of p38 mitogen-activated protein (MAP) kinase, whereas other MAP kinases and downstream targets of mammalian target of rapamycin (mTOR) remain unaffected. This study is the first to describe the effects of a P(i) transporter in cell proliferation, tumor growth, and cell signaling.

Predisposing chronic diseases and hypophosphatemia in patients with influenza

Almost half of the hospitalized influenza patients have a chronic disease, which increases the risk for secondary bacterial infections and for adults >65 years influenza is related to high mortality risk. The impact of diabetes mellitus (DM), asthma bronchiale, cardiovascular disease (CVD) and chronic obstructive pulmonary disease (COPD) on the risk of having a low serum phosphatemia (S-P) in addition to influenza is important to investigate as this increases both morbidity and mortality and can be prevented. Hypophosphatemia could be the explanation for reduced chemo-taxis and phagocytosis, which in addition to respiratory function may increase the risk of pneumonia and sepsis. Data for this study was collected from the medical journals retrospectively for 100 patients admitted to the Department of Infectious Diseases during the study period, 1992-94, with the clinical diagnosis influenza out of which seventy-two cases were used in the calculation. Forty-seven percent of the hospitalized influenza patients had a 2.7-fold risk of suffering from DM than of any other chronic disease and an almost significantly doubled risk of having a low S-P level with a chronic disease. The prevalence of hypophosphatemia (S-P<0.70 mmol/l) was high; 13.0% of the women and 15.0% of the men; 34.0% of all patients had S-P<0.82 mmol/l. Men, in contrast to women, showed clinical signs of a secondary bacterial infection more frequently (12/41 and 6/35, respectively). Our study gives indications for an involvement of low S-P with chronic disease.

Hyperventilation and hypophosphataemia

We describe a case where extensive investigations were utilized to identify the aetiology of hypophosphataemia in a patient presenting with non-specific symptoms. The diagnosis of hyperventilation-induced hypophosphataemia was eventually established. Hyperventilation is a relatively common cause of isolated hypophosphataemia, but is easily overlooked as a cause.

A case report of Wernicke's encephalopathy in a pediatric patient with anorexia nervosa--restricting type

PURPOSE

To describe a case of Wernicke's encephalopathy in a 15-year-old female patient with anorexia nervosa--restricting type.

METHOD

The patient and family were interviewed. Literature relevant to the case was reviewed prior to diagnosis.

RESULTS

The patient reported a 30-kg weight loss over a 6-month period, following a strict diet deplete in thiamine without vitamin supplementation. The patient reported a carbohydrate binge for 5 days prior to admission. Her clinical presentation, confusion, ataxia, nystagmus, and magnetic resonance imaging demonstrating bilateral subthalamic hyperdensities were consistent with Wernicke's encephalopathy. Nutritional stabilization and parenteral thiamine replacement resulted in rapid improvement of her ataxia and nystagmus and contributed to her recovery.

CONCLUSIONS

Wernicke's encephalopathy should be considered in patients with anorexia nervosa. Thiamine therapy could be considered in cases of anorexia nervosa with significant confusion.

Severe hypophosphatemia associated with gallstone pancreatitis: a case report and review of the literature

Severe hypophosphatemia (<1.0 mg/dl) is a rarely reported complication of acute pancreatitis; when it does occur, it is typically attributed to alcohol abuse rather than the pancreatitis itself (1-5). In the literature, pancreatitis is not cited as a cause of hypophosphatemia (5, 6-16). Both pancreatitis and hypophosphatemia have widespread ramifications on human physiology, affecting hematologic, neural, hepatic, endocrine, respiratory, and renal systems. Given the possible synergistic consequences of pancreatitis and low serum phosphate, we emphasize the importance of recognizing hypophosphatemia as a complication of pancreatic inflammation.Herein, we report a case of acute pancreatitis unrelated to alcohol abuse associated with severe hypophosphatemia and review the pathophysiology.

Refeeding syndrome: effective and safe treatment with Phosphates Polyfusor

BACKGROUND

Severe hypophosphataemia associated with refeeding syndrome requires treatment with intravenous phosphate to prevent potentially life-threatening complications. However, evidence for replacement regimens is limited and current regimens are complex and replace phosphate inadequately.

AIM

To assess the effectiveness and safety of 50 mmol intravenous phosphate infusion, given as a 'Phosphates Polyfusor', for the treatment of severe hypophosphataemia in refeeding syndrome.

METHODS

Patients with refeeding syndrome and normal renal function received a Phosphates Polyfusor infusion for the treatment of severe hypophosphataemia (< 0.50 mmol/L). The outcome measures were serial serum phosphate, creatinine and calcium concentrations for 4 days following phosphate infusion and adverse events.

RESULTS

Over 2 years, 30 patients were treated. Following treatment, 37% of cases had a normal serum phosphate concentration and 73% had a serum phosphate concentration of > 0.5 mmol/L within 24 h. Ten patients required more than one Phosphates Polyfusor infusion. Within 72 h, 93% of cases had achieved a serum phosphate concentration of > or = 0.50 mmol/L. No patient developed renal failure. Three episodes of transient mild hyperphosphataemia were recorded. Four patients developed mild hypocalcaemia.

CONCLUSIONS

This is the largest published series of the use of intravenous phosphate for the treatment of severe hypophosphataemia (< 0.50 mmol/L), and is the most effective regimen described. All patients had refeeding syndrome and were managed on general wards.

Why oral calcium supplements may reduce renal stone disease: report of a clinical pilot study

AIMS

To investigate whether increasing the daily baseline of gut calcium can cause a gradual downregulation of the active intestinal transport of calcium via reduced parathyroid hormone (PTH) mediated activation of vitamin D, and to discuss why such a mechanism might prevent calcium oxalate rich stones. To demonstrate the importance of seasonal effects upon the evaluation of such data.

METHODS

Within an intensive 24 hour urine collection regimen, daily calcium supplementation (500 mg) was given to five stone formers for a 10 week period during a six month crossover study. In a further population of patients on follow up for previous renal stone disease, observations were made on 1066 24 hour urine samples collected over five years in respect of seasonal effects relevant to the interpretation of the study.

RESULTS

In the group of patients on calcium supplements the following results were found. During calcium supplementation, the proportion of urine calcium to oxalate was higher (increased calcium to oxalate molar ratio), the 24 hour urine product of calcium and oxalate did not rise, and urine oxalate was lower during the first six weeks of supplementation. Twenty four hour urine calcium was 10.2% higher than baseline in the final four weeks of the 10 weeks of supplementation. Twenty four hour urine phosphate was 11.4% lower during the first six weeks of supplementation, but then rose while the patients were still on supplementation; renal tubular reabsorption of phosphate (TmP/GFR) mirrored the urine phosphate changes inversely. PTH was higher after stopping supplementation, but 1,25-(OH)2-cholecalciferol changes were not detected. In the 1066 urine samples collected over five years the following results were found. Calcium and oxalate excretion correlated positively and not inversely. Urine calcium and phosphate excretion were 5.5% and 2.5% higher, respectively, in "light" months of the year compared with "dark" months. A post summer decline in both urine calcium and urine phosphate was relevant to the interpretation of the study.

CONCLUSIONS

Regular calcium supplementation does not raise the product of calcium and oxalate in urine and the proportion of oxalate to calcium is reduced. The underlying mechanisms of the changes seen in phosphate, calcium, and PTH and the observations on 1,25-(OH)2-cholecalciferol are not clear. Observed changes in phosphate could possibly be part of a calcium regulating feedback loop operating over a period of weeks. In evaluating these mechanisms background seasonal effects are important. It is possible that "programming" of the gut mucosa in terms of calcium transport is a major determinant of the relation between calcium and oxalate concentrations in urine and their relative abundance. Increased oral calcium, in association with a reduction of the relative proportion absorbed, may be pertinent to the prevention of calcium oxalate rich stones.

Severe hypophosphatemia during hematopoietic reconstitution after allogeneic peripheral blood stem cell transplantation

A patient suffering from acute myeloid leukemia (FAB M5a) received a PBSC allograft from a matched, related donor. On day 13 after transplantation severe hypophosphatemia (0.21 mmol/l) was first noted which persisted irrespective of intravenous phosphate administration, and within 2 days reached concentrations below 0.13 mmol/l. After repeated phosphate substitution serum phosphate returned to 1.40 mmol/l on day 17. Phosphate in urine, and calcium in serum were recorded as unchanged throughout. Clinical signs and symptoms due to severe hypophosphatemia were not observed except for paresthesia in the lower extremities. The precipitous fall in serum phosphate coincided with hematopoietic reconstitution as reflected by a steep rise in leukocyte count from 0.08 x 109/l on day 10 to 5. 94 x 109/l on day 15 after transplantation. Thus, isolated hypophosphatemia was likely the result of excessive cellular phosphate uptake during hematopoietic reconstitution. Electrolyte monitoring after PBSCT should include serum phosphate to identify the hypophosphatemia associated with hematopoietic recovery.

The interaction between dietary fructose and magnesium adversely affects macromineral homeostasis in men

OBJECTIVE

Studies with rats have found that an interaction between fructose and magnesium affects macromineral metabolism; high dietary fructose significantly increased kidney calcification in both male and female rats, particularly when dietary magnesium was low. This study tests the hypothesis that an interaction between dietary fructose and magnesium adversely affects macromineral homeostasis in men.

METHODS

Eleven men aged 22 to 40 years were fed a mixed, Western diet for four 42-day dietary periods in which dietary magnesium was either approximately 170 or 370 mg/day and dietary fructose was either 4% or 20% of energy. A decaffeinated beverage containing high fructose corn syrup replaced cornstarch, bread and rice in the low fructose diet to give the high fructose diet.

RESULTS

High dietary fructose significantly (p<0.01) increased magnesium balance during both low and high dietary magnesium intakes. Ultrafilterable and ionized serum magnesium also apparently were related to magnesium and fructose intakes; they were higher when fructose was fed and when Mg intakes were high. High fructose depressed calcium balance: the effect tended to be more marked when dietary Mg was low. High dietary fructose also significantly (p<0.005) decreased phosphorous balance. Urinary phosphorous losses were significantly (p<0.001) higher when high dietary fructose was fed. High dietary fructose also increased the concentration of serum alkaline phosphatase (p<0.005).

CONCLUSION

These findings indicate that dietary fructose adversely affects macromineral homeostasis in humans and suggest further studies to see if a high fructose diet coupled with low dietary magnesium and marginal calcium leads to bone loss.

Hypophosphataemia in general practice patients

We compared plasma phosphate concentrations in general practice patients and hospital inpatients and outpatients over an 8-month period. The distribution of results in all three groups was similar and 12-16% of results were at or below 0.8 mmol/L. In general practice patients, 8.3% of results from males and 12.1% from females were below the lower limit of their respective reference ranges. Eighteen of these patients (0.2% of results) had plasma phosphate concentrations < or = 0.4 mmol/L. On follow-up, only two of these patients had any attributable cause for their severe hypophosphataemia; in the remainder, it was unexpected and unexplained. Hypophosphataemia in outpatients and general practice patients is more common than has previously been appreciated. We present a strategy for further investigation of these patients.