To determine the frequency of birth weight digit preference for infants admitted to a large neonatal intensive care unit (NICU), the scale of rounding and its dependence on birth weight, and time and the impact on prescribing accuracy.

A consecutive cohort of birth weights extracted retrospectively from a single clinical database.

Birth weights from 9170 inborn infants recorded on an electronic prescribing database admitted to NICU over 20 years.

Data are presented for the frequency of each of the possible pairs of final digits. A statistical model of digit preference assuming rounding is used to quantify the proportions rounding to specific accuracy levels. These proportions are compared between those <1000 g and those above and over the 20-year time period.

From a population of 9170 infants admitted over 20 years, there was a highly statistically significant digit bias with an increased prevalence of multiples of 100 (p<0.0001), 50 (p=0.007), 20 (p<0.0001), 10 (p<0.0001), 5 (p<0.0001) and 2 (p=0.0005). There was clear evidence of a reduced 100 g digit bias for infants 500 and 1000 g (0%) compared with those between 1000 and 4500 g (3.7%). The maximum birth weight error due to digit bias for all infants was 5%. There was clear evidence of an improvement in accuracy over 20 years.

Digit bias in birth weights over 20 years in a tertiary NICU is highly significant at the 100, 50, 20, 10, 5 and 2-digit levels. There has been a substantial improvement in the accuracy of birth weight measurements over 20 years. The likely maximum error due to birth weight digit bias is 5% and is within an acceptable tolerance for drug dosing even at very low birth weights.

The strengths of this study include 9170 birth weight measurements from a single unit entered prospectively into a neonatal prescribing database.

The impact on the degree of rounding of different birth weight groups and over time is identified.

It is not possible to investigate the rationale for rounding and digit bias retrospectively.

Accurate birth weight measurements are essential for safe delivery of care to newborn preterm and term infants especially for prescriptions. Drug-related events in the hospital setting are the highest cause of recorded errors both in the USA

There are a range of potential sources of drug errors including documentation, calculation, preparation and administration errors.

Drug errors in neonatal care are common with 3380 drug errors being reported to the UK National Patient Safety reporting and learning system between April 2008 and April 2009.

Newborn infants admitted to a tertiary neonatal intensive care unit (NICU) range in birth weight widely from under 500 to over 4500 g. Many infants require an early prescription for drugs and infusions, and the accuracy of the birth weight is critical for prevention of drug errors. Infants who weigh less than 1000 g at birth require particularly accurate weighing, and the scales used in the tertiary NICU weigh to the 1 g level. To increase the accuracy of the birth weights, nursing protocol requires deduction of the weight in grams of any item that was unable to be removed prior to the measurement of the birth weight such as an endotracheal tube inserted as part of resuscitation. The definitive birth weight recorded should therefore be at the 1 g level.

Infants with birth weights of less than 1000 g are at greater risk of drug errors as the impact from rounding or truncating effects is much greater and they are a particularly vulnerable group. Small birth weight rounding differences could potentially increase the adverse drug effects.

Digit bias and rounding have been identified in several areas of medicine

It is unknown whether a similar digit preference with rounding or truncation of birth weights might occur for infants admitted to NICU where there is an expectation of accuracy of prescription and administration of drug and fluid medications. Over the past 20 years, there have been national and local drives for quality care with improved drug safety. It is unclear whether this has resulted in an identifiable behavioural change on the accuracy of weighing and whether there has been any alteration in the frequency of rounding or truncation of birth weight measurements.

Neonatal-specific computerised prescribing systems are used in order to minimise drug calculation and prescription errors, but these rely on accurate birth weight measurements for the calculation of the dosages of drugs and infusions. The accuracy of these systems would be reduced if the recorded birth weights were significantly altered by digit bias.

The study, therefore, sets out to answer whether birth weights from a single tertiary NICU showed evidence of digit bias and if so did this vary across different weight bands groups and over time. The impact on prescribed dose error of any identified digit bias was then explored.

All infants were weighed on admission to NICU within 60 min of birth using fully calibrated Weylux 850BT/BMI class III baby scales, H Fereday & Sons, Harlow Essex, CM19 5QP, UK. These record a stable digital weight at a 1 g level but have a defined absolute accuracy of ±5 g. These scales, or their equivalent, have been used consistently over the past 20 years. The scales are regularly calibrated in line with the manufacturer's standards.

Birth weights for all infants between 500 and 4500 g admitted to NICU between June 1993 and May 2013 were extracted from an electronic patient database utilised predominantly for accurate prescribing of intravenous drugs and fluids. This system has been used for all prescriptions on NICU over the past 20 years. The electronic patient record database system was initiated in May 1993 specifically to minimise prescribing error risk. Infants born in other institutions and subsequently admitted were excluded.

A subset of infants <1000 g was also analysed. During that time, 9170 inborn infants were admitted to the tertiary NICU and 100% of birth weights were extracted.

Data extracted from the database were analysed to determine the frequency of weight measurements at 1 g intervals from 500 to 4500 g. Weights outside this range were excluded as the number of infants above and below this weight range was very small.

Birth weights were recorded with a 1 g resolution and the number of measurements with each of the possible 100 last two digits was determined and displayed graphically along with the observed/expected ratio based on a uniform distribution of digits. The significance of the peaks at multiples of 2, 5, 10, 20, 50 and 100 was determined using a Poisson regression model.

A statistical model was devised assuming that the underlying distribution of the last two digits was uniform with the number of each digit pair observed following a Poisson distribution.

There were 9170 inborn infants admitted over the 20-year period. Of these 911 (10%) were <1000 g and are considered a particularly vulnerable group. The distribution of birth weights is shown in

Number of infants by weight.

The observed number in each of the digit bands between 0 and 99 is shown in

Distributions of recorded weight measurements for infants <1000 and ≥1000 g. The horizontal lines show the expected number if digits were randomly distributed.

Modelling the rounding for the whole dataset shows that 3.2% of the time there was rounding to the nearest 100 g and 46.7% to the nearest 10 g (

Modelled estimates of the degree of rounding, showing the proportions rounding to various digits

Rounding to nearest… | All data | <1000 g | ≥1000 g |
---|---|---|---|

Per cent (SE) | Per cent (SE) | Per cent (SE) | |

100 | 3.2 (0.5) | 0.0 (1.1) | 3.7 (0.5) |

50 | 1.5 (0.6) | 4.5 (1.5) | 1.0 (0.6) |

20 | 10.4 (0.9) | 3.3 (2.1) | 11.1 (1.0) |

10 | 46.7 (1.2) | 15.7 (3.0) | 50.2 (1.2) |

5 | 8.9 (0.6) | 9.0 (2.3) | 8.8 (0.6) |

2 | 2.1 (0.6) | 8.4 (3.0) | 1.5 (0.6) |

1 | 27.2 (–) | 59.0 (–) | 23.6 (–) |

Analysis of infants <1000 g shows that there is a greater degree of accuracy with no detectable rounding to 100 g level but 4.5% were rounded to 50 g and 15.7% to 10 g. In total, 76.4% of the weights were accurate to the 5 g level compared with 33.9% for infants ≥1000 g birth weight (

In total, 4.5% of infants with birth weights between 500 and 999 g were recorded to 50 g accuracy which if we assume that this is due to rounding gives a maximum error of 5%, and 3.7% of infants with birth weights between 1000 and 4500 g had rounding to 100 g, giving a maximum percentage error of 5%.

The accuracy of weighing has increased steadily over the 20-year period (

Change in accuracy over time: modelled proportions recorded exactly, rounded by a small amount (to nearest 2 or 5 g) or a larger amount (to nearest 10, 20, 50 or 100 g).

Neonatal intensive care requires accuracy over drug dosing as small errors can potentially lead to significant adverse effects especially as they are particularly vulnerable group with immature renal and hepatic function affecting drug handling. In view of the 10-fold weight difference between the smallest and the largest newborn infants admitted to NICU, most infant drug doses are administered based on weight criteria. For infant safety, it is important to ensure accuracy of calculated doses by using reliable weight measurements. A long-standing nursing protocol has required deduction of the weight of any extraneous item not able to be removed at the time of weighing after birth. It was surprising therefore to find that our data showed that for infants <1000 g, 4.5% had their birth weights rounded to the 50 g level and 23.5% had rounding by 10 g or more. For infants ≥1000 g, more of the weights were rounded with 61.1% rounded by 10 g or more, however, the overall error level was nevertheless lower. The data also showed that there was improvement in accuracy over time with a lower proportion showing digit bias.

Many drug errors are due to poor manual calculation and there is evidence that the use of computerised prescribing can reduce errors significantly.

The formal analysis of the data was based on the assumption that there would be an equal number of birth weights in each digit weight group. Given that the range of weights (500–4500 kg) is much greater than the putative digit preferences (0–100 g), this is a reasonable approximation and only small biases in digit frequencies can be accounted for by the non-uniform distribution of birth weights, unlike the situation in other digit preference studies, for example, age.

The difference between rounding and truncation in very small infants compared with the infants ≥1000 g suggests that there is a modification in the nurses’ behavioural response to the measured weight and an understanding that rounding or truncation may influence the outcome to a greater degree in the very small infants (

It was also considered that part of the reason for digit bias in the Canadian study was the use of analogue scales and that a move to digital scales would provide an automatic increase in accuracy. Our data have been derived solely from digital readout electronic scales and yet there is evidence of digit bias and significant rounding in a significant number of cases.

It is clear from the current study that for both groups of infants there has been an improvement in accuracy of recording the birth weight and that this has been greater for those <1000 g than those ≥1000 g. This is likely to be due to progressive improvements in nurse training and to some degree the greater involvement by nurses in dose checking and non-medical prescribing

While it is clear that rounding or truncation occurs, our data show that the likely maximum error in the recorded birth weight was 5% for infants ≥1000 g. For infants <1000 g, the maximum digit bias was 50 g rather than 100 g which again gives a maximum 5% recorded weight error. However, the maximum error from rounding birth weights is within the published recommended rounding tolerances.

The authors would like to thank the neonatal nurses who weighed the infants at birth and the junior doctors who entered the birth weights onto the electronic patient record.

AJE conceived of the study, extracted the data and wrote the paper; SAR provided statistical analysis, reviewed and contributed to the final draft.

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

None.

The use of data from the database was approved by the NHS Research Ethics Committee (Haydock) 13/NW/0159.

Not commissioned; externally peer reviewed.

No additional data are available.