ORIGINAL ARTICLE

Selfmmonitoring of blood glucose from abdominal skin:

an extraordinary method A Holstein, S Hinze, U Perret, D Nahrwold, EH Egberts

Abstract A 70 year old man with type 1 diabetes self-monitored his blood glucose for 15 years by withdrawing capillary blood from the abdominal skin. The precision of this method was determined by simultaneously comparing the specific enzymatic glucose concentrations from the abdominal skin and the finger pad. Intra- and inter-method differences and the means of self-monitoring values from abdomi- nal skin and finger pad, measured by reflectometers, did not differ statistically from their corresponding laboratory monitorings. Thus, self-monitoring from abdominal skin is equal to and interchangeable with self-monitoring from finger pads, provided a high monitoring standard is given. Bootstrap simulation with another l,OOOsamples, accidentally fixed by a computer, confirmed the accuracy and reproducability of the new method.

Blood glucose self-monitoring from abdominal skin cannot generally be recommended, but after a quality check, this method might be an alternative to the well-established method of the finger pad for some people with diabetes.

Practical Diabetes Int 1998; 16(1): 12-1 5 Key words diabetes mellitus; blood glucose self-monitoring; abdominal skin; quality check

Introduction For people with diabetes, careful self-moni- toring of blood glucose several times per day is an indispensable requirement for intensive insulin therapy'. Normally the capillary blood is taken from lateral areas of the highly innervated finger pad, rarely. from the ear lobe, Unfortunately, people with diabetes often find self-monitoring painful and con- sider it a constant burden, leading to a lack of motivation and compliance.

A 70 year old patient with type 1 diabetes needed sensitive fingertips for his job. Therefore for 15 years he self-monitored his blood glucose by withdrawing blood from the abdominal skin. The precision of this method was evalu-

ated by measuring the specific enzymatic glucose concentration from abdominal skin and finger pad simultaneously

Patient and methods Patient The 70 year old patient (height: 1.7m, weight: 82kg, BMI 28.4kg/mz) had had type 1 diabetes since 1950. Apart from incipient diabetic nephropathy, no diabetes-related complications were known. He carried out intensified insulin therapy (four injections per day) with isophane insulin and diet-

related soluble (regular) insulin. His glucose control was not good (HbA,, currently 9.0%). was unstable, and there was impaired awareness of hypoglycaemia.

Working as a self-employed engineer he was in charge of the development of unique pipes, measuring their quality with his fingers. Therefore he refused blood glucose self-monitoring from his finger pads and has been withdrawing the capillary blood 6-8 times per day from various areas of the abdominal skin for 15 years. The precision of this almost painless method has not been tested previously.

Method On five successive days, up to eight times per day, we determined the glucose concen- tration from simultaneous measurements of capillary blood from abdominal skin and from the finger pad. Using a lance, the patient withdrew blood samples from a central hyperpigmented and slightly indurated area of the right mesogastrium within an area of approximately 6 x 5 cm (Figures I and 2 ) or of the lateral finger pad. He tested both samples with test strips by analysing them quantitatively with a glucometer (Gluco- Touch., Lifescan Inc.). From the same blood samples the patient put 2Op1 blood into pre-

Klinikum Lippe, Detmold, Germany A Holstein MO, RegistrarandDiabetologist S Hinze MD, SeniorHouse Officer U Perret MO, SeniorHouse Officer D Nahrwold MD, House Officer EH Egberts MD, Professor of Gasfroentero/ogyand Consultant Physician

Correspondence to: Andreas Holstein MD, Medizinische Klinik I, Klinikum Lippe - Detmold, D - 32756 Detmold, Rontgenstrasse 18, Germany

Submltted: 17 June 1998 Accepted for publication: 19August 1998

pipette haemoclastic solution using end-to- end capillaries. The glucose concentrations of these blood samples were simultaneously doubly measured by a standard laboratory method (glucose oxidase method). The results were given by the arithmetical mean of both values. The patient documented the results of his self-monitoring without being aware of the laboratory results.

Statistics All statistical calculations were done on SIMSTATO version 2.10 (1993; Provalis Research, Montreal, QC). For blood samples from abdominal skin and finger pad, specific enzymatically and self-monitored determi- nations were calculated as the means, stan- dard deviations, minimums and maximums as well as 10% and 90% percentiles. Results of both methods, from different areas (blood glucose self-monitoring from abdominal skin vs finger pad) were conelated with Pearson correlation, including the corres- ponding confidence intervals and according to the Bland and Altman methodu. This was done by applying the limits of agreement to the differences of the related measurements. According to the low size of random samples of self-monitorings and lab-monitorings, we implemented a bootstrap simulation for the gradient as well as for the area of ordinates of the linear functions. Blood glucose values from the different areas were tested with the Student t test for dependent samples. A statistical significance with a type I error (alpha) by < 5% ( p < 0.05 ) was accepted.

Results Table I shows a selected daily blood glucose

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ORIGINAL ARTICLE Self-monitoring of blood glucosefrom abdominal skin: an extraordinary method

Figure 1. Area of withdrawn bloodfrom abdominalskin Figure 2. Application of blood onto the test strip

profile. It shows self-monitoring results by the glucometer and laboratory determina- tions examined simultaneously from capil- lary blood from abdominal skin and finger pad. The high divergence (at 03.00) between self-monitoring from the abdominal skin and the finger pad cannot be explained and only occurred once. Table 2 shows basic data of all monitoring results within the five days of testing. The broad measurement area under- lines the instability of the metabolic situ- ation.

For the blood glucose measurements from self- (SM) and laboratory (LM) monitorings as well as for self-monitorings from various skin areas, a significant linear correlation for the gradient and the area of ordinates is shown (Table 3). These results were confirmed by bootstrap simulation, which also revealed close confidence intervals for gradients and segments (Tablel).

Since the formation of a linear correlation, especially in smaller, homogenic samples, is considered n~n-reliable~’, differences and means were determined intra- and inter- method (Table 5). We could not prove a statistically significant discrepancy of zero for the calculated differences (Table 6). According to Bland and Altman*”, we have to assume a sufficient correlation between two methods of measurement if at least 95% of the related discrepancies are within the mean discrepancy f doubled standard deviation of discrepancies. Up to 96% of the related

Table 1. Selected daily blood glucose profile (day 5) (mgYd1; LM = laboratory monitoring, SM = self-monitoring)

Time 03.00 06.30 09.00 12.00 15.00 18.00 22.00 I I ~ _ _ __ ~ ~~ ~ ~ ~ _ _

LM Finger 83 152 257 267 90 46 282 SM Finger 85 154 225 263 92 42 342 LM Abdomen 44 148 233 257 105 47 275 SM Abdomen 36 139 225 243 98 40 280

Table 2. Describing statistic of all bloodglucose measurements (mg/dl)

I Median 10% percentile 90% percentile Scale I LM Finger (n=26) 148 53 299 4 H 1 3 SM Finger (n=30) 123 60 317 42-342 LM Abdomen (n=27) 131 46 287 43-31 0 SM Abdomen (n=32) 124 44 298 36-369

discrepancies in our study were within this segment (Table 6, Figures 3-5).

Discussion Self-measurement of blood glucose by with- drawing capillary blood from abdominal skin is an unusual method. As far as we know, this method has been neither pub- lished nor validated previously.

Comparing two methods of clinical mea- surement is not easy. The exclusive deter- mination of the intra-class correlation CO- efficient seems to be inappropriate. We

therefore applied the Bland and Alman method, which considers differences bet- ween measurements for each subject and which is not dependent on the range of the sample2-’. This is important, especially as in small-range and homogenic random sarn- ples. the calculation of linear correlation does not provide the power.

Blood glucose self-monitoring from ab- dominal skin, as practised by our patient for 15 years, appears to be a precise method compared with the laboratory reference method. For our patient, this method is an

Table 3. Correction coefficients for simultaneous measurements

Correlation Significance Regression 95% confidence intervals coefficient (p) equation (r) Slope Intercept

y = 0.98~ +3.5 (0.85; 1.12) (-22; 29.1) LM Finger-SM Finger 0.95 <0.0001 LM Abdomen-SM Abdomen 0.99 .5O.o001 y = 1.05~-5.1 (0.98; 1.12) (-1 7.9; 7.7) SM Finger-SM Abdomen 0.95 <0.0001 y=0.96~-2.5 (0.85; 1.08) (-1 9.5; 24.5)

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ORIGINAL ARTICLE Self-monitoring of blood glucose from abdominal skin: an extraordinary method

r

Mean Standard Limits of Number of differences (mgm devlation agreement within these ilmits (%)

difference LM Finger-SM Finger -0.88 30.2 (-61.3; 59.5) 25 (96%) difference LM Abdomen-SM Abdomen -2.56 16.2 (-35.0; 29.8) 26 (96%)

27 (QOVo) difference SM FingerSM Abdomen 3.77 29 (-54.2; 61.8)

Table 4. Bootstrap resampling (No.= 1 OOO) -~ ~

Number of Slope intercept 95% confldence intervals su welllances

Slope intercept

LM FingerSM Finger 26 0.98 3.54 (0.85; 1.08) (-1 1.8; 18.7)

LM Abdomen-LM Abdomen 27 1.05 -5.1 (0.96; 1.13) (-1 5.3; 5.1)

I SM FingerSM Abdomen 30 0.96 2.5 (0.85; 1.12) (-17.9; 19.4)

Table 5. Descriptive statistic of differences and means

I No. Mean (mg/dl) t SD Standard error Range I

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ORIGINAL ARTICLE Self-monitoring of bloodglucoseji-om abdominal skin: an extraordinary method

Figure 4. Differences and means of abdominal skin-measurments and limits of agreement

a Muns (maldl) rbdamn run LM - (IM

Flgun, 5. Diflerences and means of self-measurments from fingerpadandabdominalskin with limits of agreement

blood flow after injecting the lancet, which increased the precision of the monitoring. No further manipulation was necessary.

The method cannot yet be recommended in general, since there has not been a systematic measurement in a larger group of subjects. However, it could be an alternative to withdrawing blood from the finger pad for some diabetic subjects. In any case, this new method (like conventional self-monitoring) requires appropriate training and quality control by simultaneous laboratory moni- toring to minimke possible user

Acknowledgement We would like to thank Dr Maria Tortjk from the Biometrical Institute of the Medical College, Hannover. for her help in biometrics.

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