998

PLASMA-PHOSPHATE AND TUBULARREABSORPTION OF PHOSPHATE

SIR,-It is perhaps unfortunate that in your leader onthis subject (April 18, p. 820) you did not quote the paper 1in which we advocated (a year before Bijvoet et al.2) theexpression of phosphate excretion in mg. per 100 ml. ofglomerular filtrate (PE) instead of as a phosphate/creatinineclearance ratio (Cp/Ccr), and showed that this was relatedto plasma-phosphate concentration in normal persons.

(PE is easily calculated by dividing urine-phosphate byurine-creatinine and multiplying by plasma-creatinine [allconcentrations in mg. per 100 ml.]). We suggested that thedeviation of any observed PE value from the normal meanfor the prevailing plasma-phosphate concentration mightbe termed an " index of phosphate excretion " (I.P.E.) todistinguish it from the original " phosphate excretionindex " of Nordin and Fraser,3 and we showed that thisindex yielded a very satisfactory separation on one estima-tion between the diagnostic groups studied-95 normalsubjects, 31 cases of primary hyperparathyroidism, 100cases of renal-stone disease, and 10 cases of hypoparathy-roidism.We rely on the same basic data as Bijvoet-the simul-

taneous concentrations of phosphate and creatinine inplasma and urine-which we originally introduced in1954.4 The difference between our I.P.E. and the Tm*/G.F.R. calculated by the method of Bijvoet is the differencebetween an observable fact and an extrapolation from thatfact. Our calculation simply indicates the extent to whichan observed value of PE differs from the mean value innormal persons with the same plasma-phosphate concen-tration. This is like measuring a child’s height and com-paring it with the mean height of normal children of thesame age. Bijvoet’s analysis extrapolates from the observedpoint along a theoretical line to the point where Tm phos-phate should be reached, which is rather like measuring achild’s height and then predicting his final stature fromgrowth charts.

Bijvoet and his colleagues have frequently complainedthat our indices have no " physiological meaning ". It is,however, difficult to see what physiological meaning can beattached to an imaginary Tm. We do not of coursequestion the value of an actual Tm measurement, but as faras quick methods go, our approach involves fewer assump-tions than theirs. Needless to say, it also has the two" attractive features " which you find in Bijvoet’s method-namely, the possibility of easily repeating the observationand the fact that it corrects for changes in the filtered loadof phosphate.

In practice, there will be found to be little, if any, differ-ence in the diagnostic or physiological value of these twoapproaches, since they use the same basic data, but forclinical purposes at least, our I.P.E. (which it might be moreappropriate to call APE) can be calculated by mentalarithmetic at the bedside, and does not require the use of anomogram. Blood and urine are collected in the fastingstate and the calculation is performed as follows:

Subtract 2’5 (the mean normal plasma-phosphate threshold)from the observed plasma-phosphate concentration in mg. per100 ml. and halve the remainder. The result represents the meannormal PE in mg. per 100 ml. of G.F. The APE is the differencebetween the observed and predicted PE value, and its normalrange is ±0-5 mg. per 100 ml. of G.F.* Maximum tubular reabsorption capacity.

B. E. C. NORDINL. BULUSU.

M.R.C. MineralMetabolism Research Unit,

General Infirmary,Leeds.

1. Nordin, B. E. C., Bulusu, L. Postgrad. med.J. 1968, 44, 93.2. Bijvoet, O. L., Morgan, D. B., Fourman, P. Clinica chim. Acta,

1969, 26, 15.3. Nordin, B. E. C., Fraser, R. Lancet, 1960, i, 947.4. Nordin, B. E. C., Fraser, R. Clin. Sci. 1954, 13, 477.

EFFECT OF INSULIN ON RENAL

TUBULAR REABSORPTION

SIR,-A leading article in your issue of April 18 (p. 820)refers to some work I reported to the Medical ResearchSociety. Unfortunately the quotation is not quite accurate.By maintaining a constant arterial-blood-glucose con-

centration during insulin infusion in a group of diabetics,it was possible to show that insulin reduces the amount ofglucose in the urine independently of any changes it mayproduce in blood-glucose levels. These results demonstratea previously unrecognised direct effect of insulin on thekidney, and would be in keeping with the idea that insulinincreases (and not, as you say, reduces) the renal tubularreabsorption of glucose.

G. S. SPATHIS.The Middlesex Hospital,

London W.1.

AIR POLLUTION BY NOISE

SIR,-In your otherwise excellent editorial (May 2,p. 928), you express surprise that trade-unions lack enthu-siasm for tests to identify those most liable to ear damagefrom industrial noise. When you have identified all thosewho are accident-prone, who are susceptible to industrialdermatitis, who are liable to fits, or whose hearing is mostat risk-who, then, is to employ them ? In South Wales, atleast, sheltered employment on a living wage is almostnon-existent for an unskilled manual labourer with one ofthese disabilities.

Doctors are accustomed to think of health as beyondprice, but unfortunately most people cannot afford to thinkthis. They will prefer to risk their health for the sake ofwages and job-security, until the rehabilitation and re-

employment of the disabled become a serious social

priority. In the meantime we should support unions whichpress for the elimination of bad working conditions, ratherthan the exclusion from employment of high-risk workers.

J. TUDOR HART.

FIBREOPTIC COLONOSCOPE

SlR,—I was interested in the article by Mr. Dean andDr. Shearman (March 14, p. 550). I too have been usingthe Olympus ’CF-SB’ colonoscope (I am currently usingthe new modification). I would take issue with the Edin-

burgh workers on the distance of insertion. In my experi-ence the instrument can be inserted to 40 cm. in virtuallyevery case, and the majority tolerate it to the full depth of75 cm. My technique differs slightly from that of Mr. Deanand Dr. Shearman.The patient takes three Dulcolax ’ tablets the night before the

procedure. A high saline enema is given 75 minutes before theexamination, and this is repeated 30-45 minutes later. Meperidinehydrochloride (pethidine) 50-100 mg. and, if necessary ’Valium’(diazepam) 1-10 mg. are given intravenously immediately beforeintubation to achieve relaxation and sedation. The instrument ispassed through a speculum. Once the rectum is entered, air isintroduced to distend the lumen and the instrument is advancedusing distal-tip manipulation, gentle pressure, and rotation of theprobe. At between 20 and 30 cm. the tip commonly abuts againstthe sigmoid wall.’ At this point " persuasive pressure " is broughtinto play. With this technique, steady pressure slides the instru-ment along the curve of the colon. It is essential for the mucosato be observed to slip slowly by the tip of the scope. If this doesnot occur, the fibreoptic colonoscope is withdrawn severalcentimetres, the lumen identified, and advancement againattempted. As in conventional sigmoidoscopy, the tip is directedtowards and around the valve or " angulus " of the colon to findthe lumen. Rolling the patient onto his abdomen, and occasionallyto his right side, often facilitates insertion.

Fibreoptic colonoscopes add considerably to diagnostic