General Practice

Visiting bags: a labile thermal environment

BMJ 1994; 308 doi: (Published 09 April 1994) Cite this as: BMJ 1994;308:954
  1. S V Rudland,
  2. A G Jacobs
  1. Barton Surgery, Dawlish, Devon EX7 9HQ
  1. Correspondence to: Dr Rudland.
  • Accepted 22 February 1994


Objective : To define usual colour and site of storage of visiting bags in general practitioners' cars and to investigate effect of these variables on temperature inside bag.

Design : Questionnaire to general practitioners; serial temperature measurements from paired black visiting bags at different storage sites and from bags of different colour.

Setting : South Devon coastal town during May and June. Subjects - 200 general practitioners, of whom 145 returned legible questionnaires.

Main outcome measures : Bag colour, duration and site of storage, temperature inside black bags at defined storage sites, and effects of bag colour on internal temperature.

Results : 111 (77%) of the general practitioners carried a black visiting bag, and 76 kept their bag in their car all day. The bag was coolest in the car boot, but irrespective of storage site, maximum internal temperature of the bag was always over 25°C and reached up to 80°C. Spraying a black bag silver significantly reduced the bag's internal temperature (mean difference 8.37°C (95% confidence interval 6.68 to 9.86°C) df=59, t=10.29, P<0.001).

Conclusions : General practitioners should use a silver coloured visiting bag; when visiting, they should store it in their car boot; at other times they should remove it to a cooler site.

Practice implications

  • Most general practitioners use a visiting bag to carry drugs for house calls, and this may be subjected to extremes of temperature while left in a car

  • In this study, most of the general practitioners surveyed used a black visiting bag and kept it in their car all day

  • The internal temperature of a black visiting bag stored in a car was always higher than ambient temperature and frequently exceeded drugs' thermal stability ranges

  • Spraying the case silver significantly reduced its internal temperature

  • General practitioners should put their visiting bag in the boot or on the floor of te car when visiting, remove the bag from the car at other times, and use a silver coloured bag


Home visiting remains an important component of general practitioners' work,1,2 and they must be adequately equipped with pharmacologically viable formulations to give immediate treatment if required. There isconsensus on which drugs should be carried by visiting general practitioners,*RF 3-5* but little work has been done on the temperature to which these drugs are exposed and its effect on their stability.6 Stability testing of drugs sent to Africa and Thailand stored at temperatures of up to 45°C showed a reduction in potency of both adrenaline and ergometrine maleate.*RF 7-9* During distribution in Sudan the potency of ergometrine fell by 47% and the activity of adrenaline was almost undetectable.7

In a pilot study we recorded a range of temperatures between - 4°C and 33°C in a black visiting bag inside a car over 24 hours in February. The commonly accepted temperature range quoted by manufacturers for most formulations carried by general practitioners is 0-25°C.10 We were concerned that substantial thermal damage would result from storage of drugs in cars, particularly during the summer. We therefore conducted this study to define the usual colour of doctors' visiting bags and the usual duration and site of storage of the bags in a car; to measure the temperature inside a typical visiting bag in a car; and to investigate the effect of storage site in a car and bag colour on the bag's internal temperature.

Subjects and methods Questionnaire

We sent a simple, one page questionnaire to 200 general practitioners in the area of south Devon between Exeter and Torbay. Table I shows the questions asked.


Summary of replies by 145 general practitioners*) to questionnaire about use of visiting bag. Values are numbers (percentages) unless stated otherwise

View this table:

Temperature measurements

We measured temperatures in two identical black visiting bags - made from acrylonitrile butadiene styrene and measuring 42 cmx32 cmx13 cm (Custom Cases, Hertfordshire) - inside a white hatchback car (Ford Escort) that was parked outside our surgery in an unshaded site. The maximum temperature and hours of sunlight for the recording days were provided by the Teignmouth meteorological station, five miles from our surgery. Figure 1 shows the storage sites where we measured temperatures, comparing paired sites with the two identical visiting bags. To investigate the influence of bag colour on its internal temperature we sprayed one of the two bags with silver aerosol spray paint. We then placed the cases side by side on the back shelf of the car (the hottest storage site). We compared paired data for each recording period with paired t tests.


Diagrammatic representation of storage sites for visiting bag in car

We measured temperatures with a SQ2-4U Squirrel meter/longer (Grant Instruments, Cambridge) and factory calibrated thermistors (temperature range - 15°C to 85°C, accuracy within 0.4°C). Thermistors were taped to vials containing 2 ml water - to simulate injectable formulations that were placed in slots cut in foam fitted into the top of each bag, simulating the normal storage of drugs. A 5 mm air gap between the bag's top surface and the vial prevented direct conduction of heat. Ambient temperature was recorded from a shaded location beside each bag. Internal and ambient temperatures were measured simultaneously from both bags at 10 minute intervals over about nine hours of daylight on any one day, a period similar to that which most general practitioners kept their bag in their car.



Of the 200 questionnaires sent out, 150 were returned and five were discarded because of confusing and illegible replies. Not every section was consistently answered. Table I summarises the replies.

Temperature measurements

Table II shows that the internal temperature of the bag was always higher than the local ambient temperature regardless of the site of storage (mean difference 8.42°C (95% confidence interval 7.54°C to 9.31°C), df=298, t=18.66, P<0.001). This difference was greatest on the back shelf, where the highest internal temperature (80°C) was recorded. Even when the bag was stored in the boot its internal temperature reached nearly 40°C on several occasions. Irrespective of storage site the bag's maximum internal temperature was always more than 25°C. Figure 2 shows an example of one day's recordings. Storing the bag in the car boot kept its internal temperature significantly lower than did storage at any other site except for the back floor.


Maximum temperatures recorded in visiting bags at different storage sites in car. Simultaneous recordings made at two sites for comparison

View this table:

One day's recording of temperatures of black visiting bags stored in boot or on back shelf of car

Spraying the visiting bag silver significantly reduced its internal temperature (mean difference between bags 8.37°C (6.68°C to 9.86°C), df=59, t=10.29, P<0.001; maximum difference 16°C (59°C v 75°C).


Most of the general practitioners used a black visiting bag that contained emergency drugs and was kept in their car for considerable periods. Such drugs are hotter than the ambient temperature in the car and reach temperatures that exceed their stated thermal stability range.3,10,11 This may have medicolegal implications, but, more importantly, the thermal stress may affect the efficacy of formulations such as adrenaline which are used infrequently but with the expectation of lifesaving effectiveness. The stability testing of a formulation before a product licence is granted does not seem to match conditions that the drug will commonly be exposed to. The tripartite guidelines of the International Conference of Harmonisation recommended accelerated testing of drugs at 40°C for six months and long term testing at 25°C for 12 months,12 temperatures that were exceeded in this study.

We took care to prevent the conduction of heat from the skin of the visiting bags to the vials, but this might occur if doctors were to cut deep slots into the foam filled lids of their bags. Thus, our findings may underestimate the temperatures to which vials are exposed.

Absolute temperature is not the only factor that can influence a formulation: the rate of temperature change, cyclical temperature change, and vibration will modify some preparations. Cooling a suspension can promote particle growth, and seed crystals then act as a focus for further crystallisation during each freeze-thaw cycle. Emulsions are vulnerable to creaming or flocculation at extremes of temperature, while vibration may cause coalescence; all of which can modify biological activity.

To minimise the risk of thermal damage to the contents of their visiting bag general practitioners must be confident that sample drugs have not previously been thermally damaged in a sales representative's sample case; they should keep their bag in the boot or on the floor of their car when visiting and remove it from the car at other times; and they should use a silver coloured bag, which is significantly cooler than an otherwise identical black bag.

We thank Devon Family Health Service Authority for a grant; Adrian Allsopp, Research Division, Institute of Naval Medicine, and Custom Cases, Cambridge, for providing equipment; and Julia Ratcliffe, Simon Raines, Peter Davies, Sean Tighe, Shaukat Nazeer, Colin Cable, and Pam Aires for their enthusiastic assistance.


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