The big question, is gestational diabetes misdiagnosis possible?

YES. Gestational diabetes misdiagnosis can happen. However, since setting up my Facebook support group in 2014 and having in excess of 18,000 women pass through the group, I can honestly say that we have only seen a handful of cases of false-positive gestational diabetes misdiagnosis.

When looking at gestational diabetes misdiagnosis, there are two possibilities:
  1. A false-positive diagnosis of gestational diabetes
  2. A false-negative result

First, let’s take a look at the false-positive diagnosis of gestational diabetes

False-positive diagnosis of gestational diabetes

Accuracy of the GTT

The one-hour 75 g test, used in isolation, correctly classified 85.74% of pregnant women with GDM (using the WHO criteria as the gold standard), but had a higher specificity (99.68%) and clinically very significant positive likelihood ratio (111.12)​1​

Diagnostic effectiveness of 75 g oral glucose tolerance test for gestational diabetes in India based on the International Association of the Diabetes and Pregnancy Study Groups guidelines

As with most screening tests, a false-positive diagnosis of gestational diabetes is of course possible. Research suggests this may be more common when gestational diabetes is diagnosed earlier in pregnancy with a glucose tolerance test [GTT]​2,3​.

A clinical study in 48 Chinese women who were less severe, (not in need of immediate medical attention), when comparing results of the 75g glucose load GTT twice in pregnancy (once at 24-31 weeks of pregnancy and then 2-3 weeks later), found 37.5% of these patients failed to reproduce the same result with the second GTT​4​.

In another study on 146 Japanese women diagnosed with early-onset GDM, 69 (47%) had normal 75 g GTT values in a second test at 24–28 weeks of gestation, indicating a false-positive result. Fasting levels [FPG] were significantly higher in the first 75 g GTT test than in the second 75 g GTT test. FPG levels were high in 86 (59%) women with early-onset GDM during early pregnancy but in only 39 (27%) women during mid-pregnancy​5​.

Factors that could impair the results of a Glucose Tolerance Test

The OGTT is altered by an acute episode of stress, illness, or exercise and has a high rate of intra-individual variability with a coefficient of variability (CV) of up to 5.7% of the fasting plasma glucose and 16.7% on the 2-hr sample​6,7​

A1C Versus Glucose Testing: A Comparison, Diabetes Care, and Should we continue to use the 75-g OGTT to diagnose diabetes?

Human error

Personally, I have seen more cases where women have not fasted before the first blood test, where blood has been taken before the 2-hour timescale and where there have been errors made with the recorded results, with patients data being muddled or misplaced. All these things can lead to a false-positive diagnosis of gestational diabetes.

gestational diabetes diagnosis

“Have I been misdiagnosed because my levels are all fine?”

This is something that comes up a lot.

You’re diagnosed with gestational diabetes and you start monitoring your blood sugar levels multiple times a day at home, but all your levels are BELOW the targets you have been given?! How can this be? Surely this means you have been misdiagnosed because everything is fine?

Gestational diabetes is diagnosed after having a positive result from a glucose tolerance test [GTT]. It’s important to know that you can be diagnosed with gestational diabetes from any of the blood samples taken at the GTT. A fasting level, or the post glucose level (and in some hospital trusts they test at both 1-hour and 2-hours post glucose), with any 3 of these readings being over a target threshold resulting in a positive diagnosis of gestational diabetes.

You may also be diagnosed from a single blood test, such as a fasting blood glucose test, or a *HbA1c blood test (which measures the blood glucose over a 3-month period).

*The HbA1c blood test is being used more commonly for diagnosis at the moment due to the Covid-19 pandemic meaning many hospitals have changed to this one single blood test and no need for waiting around in the clinic compared to a GTT. Read more on the Covid-19 post.

With the GTT, you are given a measured amount of glucose (in the UK and Ireland this is typically 75g of carbohydrates in the form of a sweet drink). A blood test is taken on arrival at the hospital after fasting from the evening before, then again at 2-hours after drinking the drink to see how well the body has processed the glucose load.

Gestational Diabetes OGTT Polycal drink
Polycal is one of the possible drinks used for the GTT

During this time, the patient should NOT drink water or walk around, because both of these things will help the body process the glucose and could result in a false-negative result.

A 75g carb load is a very large amount of glucose to digest at one time. It is the equivalent of 18.75 tsp of sugar! It is unlikely that you will consume this amount of sugar/carbs within your normal daily diet in one hit and/or in liquid form which is digested much faster. If you were diagnosed with a high post-glucose level, this is one reason why many may not see over target readings when first monitoring blood glucose levels, because you have not yet consumed such a high amount of carbs and your insulin resistance at this point isn’t too bad. This can soon change though! Read more here and here.

I haven’t even changed my diet and my levels are low. How can I have gestational diabetes?

Something that I see fairly often is where the mother continues with her normal diet, including some very high carb, sugar-laden foods and drinks, yet test levels all seem nice and low.

The risk of this is that blood glucose levels could be spiking before your test time, missing the spikes, with levels potentially dropping very low (also known as a ‘Spike and Crash’ or rebound hypoglycaemia). The majority of foods eaten spike to their highest point within one hour of eating, therefore it is advisable to follow a good GD diet.

Macronutrients impact on blood glucose

Missing postprandial spikes can be particularly problematic for those that have been asked to only monitor pre-meal levels, alternate days of pre-meal and post-meal levels, sporadic testing, testing only twice a day, or 2-hour post-meal testing.

National guidelines for testing

NICE Guidelines for testing​8​​​

1.3.5 Advise pregnant women with any form of diabetes to maintain their capillary plasma glucose below the following target levels if these are achievable without causing problematic hypoglycaemia:
fasting: 5.3 mmol/litre and
1 hour after meals: 7.8 mmol/litre or
2 hours after meals: 6.4 mmol/litre. [new 2015]

NICE Guidelines NG3 Diabetes in Pregnancy

SIGN Guidelines for testing​9​

Postprandial glucose monitoring should be carried out in pregnant women with gestational diabetes and may be considered in pregnant women with type 1 or 2 diabetes.

…aim to achieve blood glucose: between 4 and 6 mmol/l preprandially, and <8 mmol/l one hour postprandially, or <7 mmol/l two hours postprandially >6 mmol/l before bed.


HSE Guidelines for testing​10​

5.3.6 Blood glucose targets during pregnancy. The following target values are recommended for optimum maternal and fetal outcome: – Fasting capillary glucose level: 3.5-5.0mmol/L – 1-hour post-prandial capillary glucose level: <7.0mmol/L

HSE Guidelines for the management of GDM

It is worth noting that not one National guideline recommends only preprandial (pre-meal) testing, or alternative days of preprandial and postprandial testing. They all recommend fasting and postprandial testing, with additional preprandial testing recommended for those using insulin therapy

One hour postprandial testing versus two hours

Many studies have shown that postprandial hyperglycaemia is a predictor for fetal macrosomia and may contribute to neonatal hypoglycaemia. Current recommendations state that tests should be performed at either one or two hours post meals. Studies have
demonstrated, however, that the 1hour postprandial test is more likely to detect abnormal values which may require treatment and helps the person understand the relationship between food and blood glucose levels

NICE Guidelines NG3 Diabetes in Pregnancy

The GD diagnosis postcode lottery

Diagnostic test targets being used can differ in each area of the country and with each NHS Trust. Therefore you could be diagnosed with gestational diabetes if you live in Scotland where they are following the SIGN Guidelines with a fasting level of 5.2 mmol/L, but would not be diagnosed with that fasting level if you live in England where the NICE Guidelines are used. Likewise, you could be diagnosed with gestational diabetes with a 2-hour post-glucose level of 7.9 mmol/L in England, but get a negative result in Ireland if they are using the HSE Guidelines.

This obviously can cause much confusion over a positive diagnosis of gestational diabetes.

GD diagnosis postcode lottery

Increasing insulin resistance

Another important point to note is that insulin resistance worsens as the pregnancy progresses. Therefore, many will monitor blood sugar levels and be fine to start with, but as the pregnancy progresses, they start to see higher levels.

The time where pregnant women are most insulin resistant is around 32 – 36 weeks. This is something commonly seen in members of the Gestational Diabetes UK Facebook support group, with many being told they are ‘borderline’. To see comments from members who have been told they have a borderline diagnosis, take a look at this article.

Challenging a false-positive diagnosis

If you believe you have been misdiagnosed with gestational diabetes then it is best to discuss this with your antenatal diabetes team. Follow a good GD diet, avoid complicating things further by eating or drinking high carb and sugary foods just to ensure you aren’t missing big spikes in blood sugar levels and monitor your blood sugar levels at home. These levels can then be reviewed by your diabetes medical professionals when discussing a possible false-positive diagnosis.

gestational diabetes

False-negative results

More so now than previously, due to changes in many hospitals testing procedures because of the Covid-19 pandemic, the gold standard GTT is not being used in some areas due to the length of time required to wait in hospital between blood tests. This has meant we have seen an increase in women being given HbA1c blood tests, or random or fasting blood glucose tests.

Unfortunately, these tests result in more false-negative results. How do we know this? Because many women who have had gestational diabetes in previous pregnancies monitor their blood sugar levels from the start of their next pregnancy, see higher than desired levels, yet on completing a HbA1c, random or fasting blood glucose test they are given negative results.

The fasting or random blood glucose only measures the amount of glucose present in the blood at that one time, therefore gestational diabetes can easily be masked.

With a HbA1c blood test, it is an average blood glucose over 3 months. This means that unless blood glucose levels have been on average higher than recommended throughout this time, then a negative result will be given.

Research also suggests that the HbA1c, fasting and random glucose tests are not as accurate for diagnosing gestational diabetes​12–17​.

The importance of gestational diabetes diagnosis

GDM is associated with a trilogy of risks. Significant pregnancy complications including increased perinatal morbidity and possibly mortality can occur (4,5). A diagnosis of GDM also identifies a mother at high risk for the future development of type 2 diabetes (1). The effects of maternal hyperglycemia (of any kind) are associated with the development of metabolic problems including type 2 diabetes in the offspring (6). It is, perhaps, for this effect of intrauterine programming that the disorder is most worthy of detection. It has now been demonstrated that the treatment of GDM improves pregnancy outcomes. In the Australian Carbohydrate Intolerance Study in Pregnant Women (ACHOIS), the incidence of serious perinatal complications (a composite of death, shoulder dystocia, nerve palsy, and fracture) was 4% among women randomized to routine care compared with 1% among the intervention group (5). The number of GDM cases that needed to be treated to prevent one serious perinatal complication was 34. This indicates that excess serious perinatal complications will occur in 3% of cases of untreated or unrecognized GDM.​2​

Point: Universal Screening for Gestational Diabetes Mellitus, Diabetes Care


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