In patients with type 2 diabetes, tight control of blood glucose reduces complications and improves outcomes. As a result, increasing numbers of elderly patients with type 2 diabetes are being advised to measure blood glucose at home. For these elderly patients, who are at risk to have impaired cognition, vision, and dexterity, it is especially important that their home glucose meters provide accurate, reliable results and are simple to operate. We report a case of an elderly patient who suffered falsely elevated home glucose measurements for an interesting and avoidable cause.

An 89-year-old man with type 2 diabetes had been managed for many years with low daily doses of insulin. After an adjustment of his insulin schedule, he suffered an episode of symptomatic hypoglycemia. Because he was on small doses of insulin and his glucose control had been excellent (glycohemoglobin 6.0%), he was advised to stop all insulin administration and to carefully monitor blood glucose levels at home for a period of time.

Three days after receiving this advice, his wife called the office nurse, stating that earlier in the day the patient had a blood glucose level of 561 mg/dl, without any other untoward effects. She “sent him outside to work it off,” and when he came back his blood glucose was 175 mg/dl. They were advised to continue monitoring the blood glucose.

Two days later, the patient’s wife again called to state that earlier in the day he had a blood glucose of 591 mg/dl, and again she advised him to increase his exercise. His blood glucose 2 h later was 180 mg/dl. The patient and his wife were asked to come to the clinic and to bring his home glucose meter for further investigation of these wildly fluctuating blood glucose measurements in a previously stable individual.

During the clinic visit, it was determined that the home glucose meter was working properly. The patient’s 56-year-old daughter, an accountant, arrived in the meanwhile to state that she had discovered the problem. Inspecting a digital display of a calculator, she determined that the number 165 read upside down was 591, and the number 195 read upside down was 561. She concluded that the patient and his wife had been reading the glucose meter upside down.

By using a commercially available digital calculator, we have determined that the digits 0, 1, 2, 5, and 8 appear as the same numbers whether read right way up or upside down. The digit six may appear to be a nine when read upside down, and the digit nine may appear to be a six when read upside down. Thus, combinations of these digits result in the potential for a patient to turn a home glucose monitor upside down and obtain a false glucose reading.

Many commercially available home glucose meters will provide digital readings of up to 600. Thus, glucose values in the 100s, 200s, or 500s could represent the patient’s actual blood glucose at the time, or they could represent the upside down reading phenomenon.

Patients should be clearly informed which is the top and which is the bottom of their glucose meters, and companies should be sure that their devices are very clearly labeled in this regard to prevent incidents similar to the one reported here. In addition, health care personnel who deal with patients with diabetes should be aware of this potential problem in interpreting home glucose meter readings.

Address correspondence to David E. Steward, Professor and Chairman, Department of Medicine, Southern Illinois University School of Medicine, P.O. Box 19636, Springfield, IL 62794-9636. E-mail: dsteward@siumed.edu.