Three months with the Medtronic 640G – time for a review and comparison with the Spirit Combo that I used previously

Having now used the 640G for three months, I felt that I now have a good handle on it, how it works and how it compares to the Accu-Chek Spirit Combo that I was using previously.

To summarise it, as a pump, it does little differently, aside from a less functional remote control, however it has a few bells and whistles, that, for me at least, make it an amazing option. But if you want to know more, then read on.

The 640G – Form and Function as a pump

In May, I got my hands (finally) on one of these, being funded by the NHS:

Previous to this, I’d spent about 7 months with a purloined Combo, and self funded the sets (although a number of helpful people provided me with plenty as well). That’s one of these:


On a basic level, if I was to make a comparison, in terms of “On-Pump” functionality, I think the combo and the 640G both do very similar things. They have the same range of pump functionality, in that they both offer multiple basal rates, have normal, square and dual/multiwave bolusing, and do what pumps do. Both also offer a form of bolus wizard that allows you to enter a load of data and then have the system calculate what your bolus should be. Whilst the functionality is the same, the “how” the functionality is delivered is very different. Why? The remote control.

As everybody knows by now, the Spirit Combo includes a linked Aviva “Expert” meter (which for many is a killer feature of the pump), on to which all the Bolus Wizard data is entered. Then when you blood test, the bolus amount is sent to the pump to deliver. In the case of the 640G, whilst you do the blood test on the Ascensia Contour Next-Link 2.4 (bit of a mouthful that), and the test result is sent to the pump, you enter all the data on the pump, which is where it does the calculations.

The other thing that the meter has is a full emulator of the pump functionality, so that, if you wish, you can hide the pump away and run it all remotely. This is not something that you can do with the 640G. The only option there is to offer a remote bolus, which is similar functionality to the old “bolus button” you could get for the Veo pumps.

For many, this is to the detriment of the 640G. To me, however, it’s not. Why not? Simply because after a few weeks, the novelty of the remote had worn off and it was generally more expedient to use the pump to do things. The mental arithmetic required to calculate bolus amounts isn’t that hard, and given I ignore anything anyone says to me about insulin stacking (as my experience indicates that the relationship between insulin dosing is additive rather than multiplicative, understandably), it really doesn’t seem to make any difference to understand the IOB, except in very specific circumstances. The Aviva didn’t take this into account anyway, so I’d rather not carry the lump around with me when using it was of limited value.

The only use for remote bolusing option was really when I was at a black tie dinner and the pump was under a cummerband. At that point, once, it was useful.

It’s therefore safe to say that a remote control is not high on my list of priorities. Very much a nice to have rather than a must have.

Physical properties

Now, comparing the pumps themselves, the Combo is smaller and lighter than the 640G, but not by much. Empty, there’s 15g in it, and in terms of dimensions, the Combo is 82.5 x 56 x 21mm while the 640G is 53 x 96 x 24mm. The 640G is about the same size as the Animas Vibe, and of course it’s also hiding all the radio equipment for the Contour and the Glucose sensor. When you’ve put a skin or case on the Combo to get a belt clip, they both feel about the same.

As I wear the pump on my waistband or belt, it made little difference to me. If you were wearing it in your bra, as many women do, you would be more likely to feel the difference.

Both pumps use a 3ml reservoir, although a smaller one is available for the Medtronic, if you prefer.

I think physical controls on the 640G are easier to use, but that links to the interface, so can’t be fully separated. The screen on the 640G is, however, massively better, being both bigger and colour (and also usable in sunlight).

A frustration with the 640G is that when wearing it on your belt, it has only one orientation. That is one where the exit of the reservoir points upwards. On the combo, you can point it in any direction. Upwards is daft given the risk of air bubbles in the reservoir. Having said that, I’ve not experienced any issues with air bubbles affecting flow, so while it may appear to be an issue from a superficial perspective, I’m not sure it’s as big an issue as I seem to think.

The other major difference between the two is that the 640G is waterproof. How much? Fully. I spent a week on holiday jumping in and out of the swimming pool to no detriment, completely able to forget about it. That’s a huge bonus in my book, and gives it some major plus points.

On the physical side then, given the comparison between the two, the better screen and waterproofing, in spite of the larger size, wins it for me.

User Interface

While you can hardly call the interface basic on the Combo, I don’t think it’s as slick as that on the 640G. Due to the control mechanism on the Combo, the interface is linear, which I found very frustrating. It was easier with the remote, but still not ideal.

The 640G is a more recent pump and takes advantage of the advances in tech to provide something that’s a more up to date.

In reality, they both provide the expected data relating to history, etc., and allow access to things like event reminders, such as allowing yourself to set blood testing alarms, set change alarms, and the like. You are also able to disable various alarms, vibrations, etc in both.

One of the things I like is that the 640G doesn’t alarm when you complete a TBR. This is extremely helpful overnight as it doesn’t wake me, and more importantly, my partner.

The thing that I don’t like is the number of clicks required to issue a bolus. this is subtly different between the two. On the Combo, you select the bolus type up front then enter amount (then duration and split if necessary) and hit go. On the 640G you enter the total amount, then elect whether it is normal, Dual or Square. Then you determine split and then you hit go. It’s slightly more convoluted, and easy to forget the last step “Deliver bolus”. Especially if you are rushing.

On the other hand, you do have the ability to stop a bolus if you make a mistake. This is a big benefit compared to the combo.

All in all, for me the newer pump interface is the easier to use.

Overall then. in terms of form, I prefer the 640G. In terms of pump functionality, there is very little to choose between the two.

The bells and whistles…


What do I class as bells and whistles? For the Combo it’s that remote control. Full control of your pump without using the pump. It’s handy, no doubting that, but for me, as I mentioned, it wasn’t a deal breaker. After a month or so of using it, I stopped. I didn’t want to rely on another item that I had to have out and available for use.


For the 640G? It’s the integrated CGM and the SmartGuard features. These are as close as you are going to get to an Artificial Pancreas in a commercially available pump system (until the 670G becomes available, probably next year). This is very much a unique feature of the 640G and one about which I’ve already written. It stands the pump head and shoulders above anything else on the market, in my view.

640G – CGM and SmartGuard

So let’s have a quick review of it, and the much ill-regarded Enlite system on which it relies. I entered into using the SmartGuard system having read some terrible things on the web about the efficacy of the sensors, the failure rates that people saw, the MARD versus other CGM systems and I’d be lying if I said I wasn’t a little concerned about this stuff. In spite of this, I elected to jump in anyway.

It goes without saying that I’m also lucky enough to be able to afford to fund Enlite sensors. They aren’t cheap. With the deal, it works out at around £145 per month for sensors alone, as long as you extend them. I’m willing to pay that as I think that CGM provides me with enormous benefits and I’d rather manage my diabetes this way. I think this should be NHS funded as it drives a huge part of this pump’s functionality.


Placing the sensors on my arm, rather than the recommended location of the stomach area, as I had done with both Dexcom and Libre sensors, I think has been the saviour of this system for me.

From a completely personal use standpoint, the performance of the CGM on the system has been no worse than Dexcom or Libre. If anything, it’s better than Libre. I’ve found that the Enlites paired with the updated Guardian transmitter have tracked accurately enough and I’ve been confident in what they’ve provided. To the extent that on occasions, I’ve chained CGM readings to calibrate the system when blood testing wasn’t an option. I had been using the xDrip system rather than an official Dexcom receiver previously, so I’d been able to manage with fewer calibrations when I needed to.

All in all, from a CGM perspective, it’s worked well.

While the Enlite sensors really don’t last as long as Dexcom (I’ve stretched an Enlite to 12 days, whereas my longest on Dexcom was 29) I was finding that Dexcom after 14 days wasn’t as reliable as I’d like. I certainly wouldn’t run any Artificial Pancreas software on a Dexcom sensor that was older than 14 days. I’ve been running SmartGuard on the 6-12 day old Enlites with no issues. From a use case scenario, then, it really depends on what you’re trying to do with it.

I’ve seen limited failures on the Enlites (apart from at the start, where I had bleed issues and managed to forget to press the right button on the Serter, ripping the sensor out and rendering it useless immediately after inserting). I’ve not found the issues that seem to be found across the board raising their head. Maybe it’s just a matter of time, but I’m happy so far. I also like having the CGM and pump in a single device. The less I have to carry about, the better really. All we need is the Connect functionality so it can talk to a phone. Come on Medtronic. When’s this coming?


If there was a pump killer app, next to full APS functionality, this is it. I’ve previously about how it works and my experiences, and you can find that here. Suffice to say, for a review, I think it’s brilliant. The “Suspend before low” tends to kick in at pretty much the right time nearly all the time. It means you can change the behaviour of how you bolus for various food types and if you were hypo unaware, it would make a tremendous difference to your life. You could set it in such a way that you’d pretty much never have a hypo. I think it’s that good.

But instead of spouting on, let me show my last 15 days averages:

That’s right. 3% of an average day below 3.8 (I consider 3.5 to be truly hypo, but start to feel them around 3.8) and 17% above 8.8. We’ll discuss that second stat in a moment, but that level of lows is fantastic. I haven’t really suffered debilitating lows as an issue in my life, but I did have issues with overnight lows occurring after the gym. This has more or less removed that. It’s really quite stunning.

The second is the lack of time spent above the target range. Why? My take on this is that with the basal set up correctly, and the SmartGuard in operation, I feel completely comfortable in bolusing more aggressively, especially when in the context of eating at short notice and consuming higher levels of carbs.

The data in the above picture comes from a period which include low carbing, but a significant proportion on holiday in Ibiza, where eating was wholly in restaurants guessing carb contents and nights out that included Espresso Martinis, which are fairly sweet drinks. So it’s by no means an unrepresentative set of data.

So, an 80% time period in target range. Okay. That’s not perfect, but it’s still really good. And if we were to use the normal statistic, where the target range is below 10mmol/l, that period above range would drop to 8%, giving a 89% in range!

To sum up then, I’m hugely impressed with SmartGuard. By setting it at appropriate levels you can not only eliminate hypos from basal related issues that occur, but I’ve also found I can bolus far more aggressively, and still not go hypo. This is amazing stuff!


To conclude then, in the three months I’ve had it, I’ve found the 640G to be a great pump. It’s not my first, so I’m not coming at this with no comparisons. I’m thoroughly grounded in many of the diabetes technologies, and I’m very impressed with this one. So, my bullet points below, for what I consider the pros and cons of this device.


  • Pump form – I like the screen, layout and how it looks and feels. To me it feels more modern than most pumps on the UK market. If I’d held the T-Slim, I might think differently.
  • User Interface. I like it. I know a fair few don’t.
  • Waterproofing. All pumps should be fully waterproof.
  • SmartGuard – the knock out blow as far as I am concerned. There is nothing else like this on the market.
  • Unnecessary extra button presses and some of the logic of the UI.
  • The lifespan and cost of Enlites.
  • No Connect functionality to link the Pump CGM to a mobile phone.
  • Reservoir orientation is always up on a belt. Whilst it hasn’t affected me, this doesn’t feel like clever design!

Maybe I’ve got a good example here, but I’ve not run into any of the issues that I know have been seen amongst 640G users, to such an extent that it’s been labelled the 666G. I’m still getting on very well with it and see no good reasons in my use of it to think otherwise. It maybe that, adopting a product like this a year after it has come out has given the manufacturer enough time to iron out some of those wrinkles.

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