the management of diabetes has seen changes in the past 30  years , which have improved the lives of patients afflicted with this disease . improvement in glucose measurement started with the introduction of selfmonitoring of blood glucose ( smbg ) in the late 1980s , which was one of those important changes . as the second device for measurement of blood glucose level , the continuous glucose monitoring system ( cgm ) was introduced to the market around the beginning of this century . it provides maximal information about fluctuations in blood glucose levels throughout the day and optimal treatment decisions to control diabetes . in 1999 , the food and drug administration approved the cgm device in the usa . since then , the device has been improved dramatically , and the realtime cgm and insulin pumpintegrated system are already available commercially in the usa and europe . in japan , the history of cgm is relatively short . the ministry of health , labor and welfare approved the cgm device in 2009 , approximately 10  years after the usa . furthermore , the secondgeneration blind cgm , the minimed gold of medtronic , has been the only device approved under the governmentsupported medical insurance in japan . a new wireless , but blind cgm , is going to be in japanese market in a very near future , at last . with this different historical background , cgm has already been studied extensively in the usa and europe . at this stage , it is necessary for japanese clinicians to know what has already been investigated . in particular , we need to understand whether the continuous glucose monitoring system really is an accurate , safe and clinically effective device , or not . in the present article , we review the literature and introduce some useful information on diabetes care in the clinical setting especially to the physicians in countries with short history of cgm . the diabetes control and complications trial ( dcct ) confirmed that patients treated intensively with insulin show better results with regard to the prevention of diabetic microvascular complications than those treated conventionally . however , we also know that the intensive insulin therapy enhances the risk of hypoglycemia . patients who experience frequent episodes of hypoglycemia often develop various abnormalities and lose their ability to detect hypoglycemia . hypoglycemia unawareness and it perpetrates  a vicious cycle of recurrent hypoglycemia. however , there is strong evidence showing that minimization of the hypoglycemic episodes through more meticulous glucose control can actually reverse many of the counterregulatory defects . for meticulous glucose control and prevention of hypoglycemia , the cgm is a useful device , as it provides maximal information about fluctuating blood glucose levels throughout the day . the finger stick test provides only information on blood glucose at one point in time , so even if  hypoglycemia unawareness occurs between the measured points , we are not able to detect it . with the cgm , we might accomplish both purposes  better control with the intensive insulin therapy and prevention of hypoglycemia unawareness. the cgm is a method that provides constant checking of blood glucose level through measurement of glucose concentrations in the interstitial fluid . the minimed gold of medtronic , which is currently available and used in japan under the governmentbased health insurance system , features a sensor inserted in the subcutaneous tissue just under the skin of the abdomen . the sensor comprises a flexible , platinumplated electrode housed inside a permeable membrane , which can be used for up to 72  h. subcutaneous glucose level is measured by the glucoseoxidase method , and the interstitial glucose level measured every 10  s is sent to a monitor that shows the average blood glucose value every 5  min . the minimed gold calculates blood glucose values in the range of 40400  mg / dl . athough the user has no access to realtime glucose levels , the measured data can be downloaded into a spreadsheet . this is carried out at least four times a day in the minimed gold , using the finger stick test . it means that selfmonitoring of blood glucose is closer to real glucose levels , the venous glucose levels , at any one point in time . it should be noted that the device has a time lag between blood and interstitial glucose levels . there was no information on the performance of the sensor during sustained hypoglycemia or during recovery from hypoglycemia at that time . venous blood glucose levels were maintained at euglycemia for the first 60  min , then fell to 45  mg / dl for 60  min , but were finally restored to euglycemia . the sensor profiles paralleled blood glucose levels at each of the three plateaus with a correlation coefficient of 0.79 , and mean absolute error of 7% . the drop in glucose level measured by the sensor closely matched the drop in blood glucose , but the recovery from hypoglycemia was delayed by an average of 26  min . the device was the firstgeneration minimed cgm , which was probably less accurate than the secondgeneration , minimed gold . in 2003 , boyne et  al . tried to measure a time lag between the firstgeneration minimed cgm and blood glucose , and also between the different sensors . a total of 14 patients with type 1 diabetes each had two sensors placed subcutaneously in the abdomen , acquiring data every 5  min . at the same time , blood glucose was sampled every 5  min for 8  h. the results showed that the  time differences between blood and interstitial glucose levels ranged from 4 to 10  min , with the interstitial glucose lagging behind blood glucose in 81% of cases . the mean (   sd ) difference between the two sensors in each patient was 6.7    5.1  min , representing random variation in sensor response. the authors also showed that the lag times were statistically significant for the rise ( 10.1    10.1  min , p  <  0.001 ) , fall ( 6.9    8.5  min , p  =  0.017 ) and nadir ( 9.4    7.7  min , p  <  0.001 ) in glucose levels . in each case , blood glucose level preceded that of interstitial glucose . other groups reported similar results , although there was some variability among the different studies . thus , the main conclusions were that there is a time lag between the measured interstitial glucose levels and actual blood levels , and that the different sensors have different sensitivities in measuring interstitial glucose levels . the clarke error grid analysis ( ega ) , designed by william l clarke , was first reported in 1987 . the ega is designed for smbg to evaluate the agreement and discrepancy between blood glucose values and sensor readings at isolated static points in time . this method was innovative , because it took into account not only the difference between the systemgenerated and reference blood glucose values , but also the clinical significance of this difference . in other words , clarke et  al . tried to evaluate the accuracy and clinical significance of the continuous glucose monitoring system . in 2004 , they also reported an improvement to the original ega , and introduced the cgega , the continuous glucoseerror grid analysis . cgega was specifically designed to evaluate the clinical accuracy of continuous glucose monitoring in terms of precision of both blood glucose readings and blood glucose rate of change . temporal characteristics of the data , analyzing pairs of reference and sensor readings as a process in time represented by a  bidimensional time series and taking into account inherent physiological time lags . in this method , they introduced a new concept of  rateerror grid analysis ( rega) in addition to modifying the traditional ega into a new  pointerror grid analysis ( pega) that reflects the temporal characteristics of blood glucose . the rega is a rateerror grid analysis that assesses  the sensor s ability to capture the direction and rate of blood glucose fluctuations. for each pair of rbg ( reference blood glucose ) readings ( rbg [ t1 ] , rbg [ t2 ] ) taken at times t1 and t2 , the rbg rate is computed as bg divided by the elapsed time . the rbg rate of change ( mg / dl / min )  =  ( rbg [ t2 ]    rbg [ t1])/(t2 similarly , for each sensor blood glucose ( sbg ) pair ( sbg [ t1 ] , sbg [ t2 ] ) , sbg rate is computed as sbg rate of change ( mg / dl / min )  =  ( sbg [ t2 ]    sbg [ t1]/[t2    t1 ] ) . then , the sbg rate is plotted against the rbg rate ( figure  1 ) . the pega is a pointerror grid analysis that evaluates the sensor s accuracy in terms of  correct representation of blood glucose values. point accuracy reflects the difference between two paired samples at one point in time ( figure  2 ) .  the rateerror grid analysis ( rega ) divided into ar , br , cr , dr and er for sensor blood glucose ( sbg ) rate vs reference blood glucose ( rbg ) rate . the pointerror grid analysis ( pega ) divided into ap , bp , cp , dp and ep for sensor blood glucose ( sbg ) vs reference blood glucose ( rbg ) . the pega zones are defined based on the reference rate of changes in blood glucose . both the rega and pega divide the glucose rate or glucose ranges into clinically meaningful zones : zone a , corresponding to clinically accurate reading ; zone b , corresponds to benign errors ; zone c , signifies overcorrection errors ; zone d , indicates failure to detect clinically significant rate of change in blood glucose ; and zone e , indicates an erroneous reading. the pega zones are defined depending on the reference rate of bg changes . the cgega recognizes that the clinical meaning of rate accuracy depends greatly on the absolute blood glucose level , with different blood glucose levels requiring different interpretations of the combination rega  +  pega . for this reason , the cgega computes the combined accuracy of rega  +  pega in three clinically relevant regions : hypoglycemia ( blood glucose 70  mg / dl ) , euglycemia and hyperglycemia ( blood glucose > 180  mg / dl ; figure  3 ) . as the cgega is intended for software application , most of these parameters could be user selectable . for example , the time lag between blood and interstitial glucose has a default value of 7  min , based on literature data . if a device has a longer technical lag , then the software would allow the time lag used by the pega to be changed . the continuous glucoseerror grid analysis ( cgega ) computes the accuracy of the combination of rateerror grid analysis ( rega ) plus pointerror grid analysis ( pega ) into three clinically relevant regions : hypoglycemia ( blood glucose 70  mg / dl ) , euglycemia and hyperglycemia ( blood glucose > 180  mg / dl ) . used the cgega to evaluate the continuous glucose monitoring system , therasense freestyle navigator . using the cgega , they reported that the accuracy of the navigator , measured as a percentage of accurate readings plus benign errors , was significantly different at hypoglycemia ( 73.5% ) , euglycemia ( 99% ) and hyperglycemia ( 95.4% ) . wentholt et  al . explored the cgega by comparing it with classical accuracy assessment methods , such as correlation , linear regression and mean absolute difference ( mad ) , using data reported in a previous study that compared two different continuous glucose sensors in type 1 diabetic patients . they found differences in the accuracy of mad and cgega , and that the cgega was not reliable . in contrast , clarke et  al . responded by showing that the methods used by wentholt unfortunately failed to take into account the basic structure of continuous glucose monitoring data , which represents a time series analysis . they reported that it is impossible to compare the different methods altogether , and approached the issue of accuracy by applying several different methods for  each different purpose . using the different methods complementarily is the most effective way to prove the accuracy of the device . the investment in cgega , which has been widely used by scholars until now , is really meaningful for assessing the accuracy of cgm . by using multiple established assessment methods for evaluation , we can reach the true accuracy of cgm , thus ensuring the safety and clinical effectiveness of the device . in the symposium , wolpert published some important points . as we already know , there is a lag time between interstitial and blood glucose . the lag time ranges from 5 to 15  min depending on the rate of glucose change . increasing glucose levels are reflected first in capillary blood . however , detecting decreases in glucose after insulin injection or exercise might be confounded by the placement of the cgm sensor . for example , if the cgm sensor is placed next to an insulinsensitive tissue , the glucose level might decrease first in the interstitial fluid before it is reflected in capillary blood glucose . also , wolpert pointed out that the sensor needs to be calibrated when glucose is  in a steady state. if the patient carried out the calibration when glucose is rising at 3  mg / dl / min and there is a 10min lag , then there will be a discrepancy of approximately 30  mg / dl between the capillary glucose measurement and the interstitial glucose measurement . another important point relates to the education of the patient . in the usa and europe , the realtime cgm are already available on the market , and many articles have been published on the device the realtime cgm wireless type is a very convenient and useful device , but it can potentially lead to excess treatment based on the measured data . if patients overcompensate for rising glucose levels using frequent bolus injections of insulin , they are at increased risk of hypoglycemia . also , patients could reduce or discontinue basal insulin therapy in order to avoid hypoglycemia . this occurs more commonly in patients who lack the understanding of the concept of the lag time between capillary and interstitial glucose , thus considering their glucose to be much lower than it actually is . we need to educate these patients about insulin pharmacodynamics , as well as the factors that affect postprandial glucose patterns . they must also be taught not to rely on interstitial glucose data alone to assess hypoglycemia . a group at the mayo clinic concluded that the device was associated with a significant reduction in mean glycated hemoglobin ( hba1c ) . it was true for adults with type 1 diabetes as well as type 2 diabetes . there was no significant difference in hba1c reduction between studies of realtime vs nonrealtime devices , if the devices were appropriately used . the metaanalysis by mayo clinic is important , because it was newly published in 2011 , and the number of the trials providing the data , 19 , was the biggest among the metaanalyses about cgm . data of adverse events were available only in a few trials and comprised mainly mild reactions , such as redness and itching , at the sensor implantation site . the device errors of data storage , and alarms going off infrequently occurred . in october 2011 ,  continuous glucose monitoring : an endocrine society clinical practice guideline was published . even if there are various conclusions in metaanalyses about cgm , the task force evaluated  three potential uses of cgm : ( i ) realtime cgm in an adult hospital setting ; ( ii ) realtime cgm in children and adolescent outpatients ; and ( iii ) realtime cgm in adult outpatients . the authors used the available data to develop evidencebased recommendations for good control of glycemia and limiting the risk of hypoglycemia . in that guideline , an alternative for patients who can not safely and effectively take advantage of the information provided to them by realtime cgm , even though there are some studies concluding that there was no significant difference in hba1c reduction between realtime vs nonrealtime devices . however , it might be an appropriate statement in countries where the realtime cgm is the first choice cgm . by reviewing the literature on cgm , we understand that this device has the potential to improve diabetes care in the clinical setting , provided we take into consideration its pros and cons . there is a need for more knowledge about its accuracy , safety and clinical effectiveness through daily medical examination . furthermore , the introduction of the more developed devices , such as realtime cgm available in the usa and europe , under the japan governmentsponsored healthcare system should be encouraged .