Lactose Intolerance Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is Lactose Intolerance?

Lactose intolerance is a common digestive condition in which the body is unable to fully digest lactose, the primary sugar found in milk and dairy products. This occurs due to a deficiency of lactase, the enzyme produced by cells lining the small intestine that breaks lactose down into its two component sugars, glucose and galactose, for absorption.

When lactase is insufficient, undigested lactose passes into the large intestine, where gut bacteria ferment it, producing gas (hydrogen, methane, and carbon dioxide), short-chain fatty acids, and water. This fermentation process causes the characteristic symptoms of lactose intolerance: bloating, abdominal cramps, flatulence, and diarrhoea, typically occurring 30 minutes to 2 hours after consuming dairy products.

Lactose intolerance is remarkably common worldwide, affecting approximately 65-70% of the global population. Prevalence varies dramatically by ethnicity: it affects over 90% of East Asian populations, 60-80% of African and South Asian populations, but only 5-15% of Northern European populations, where a genetic mutation allowing continued lactase production into adulthood (lactase persistence) became widespread thousands of years ago through natural selection.

There are several types of lactose intolerance:

• Primary lactose intolerance: The most common type, caused by a genetically programmed decline in lactase production after weaning. This is the normal pattern for most humans worldwide.
• Secondary lactose intolerance: Caused by damage to the small intestinal lining from conditions such as coeliac disease, Crohn's disease, gastroenteritis, or chemotherapy. This type may be reversible once the underlying condition is treated.
• Congenital lactase deficiency: An extremely rare inherited condition where infants produce little or no lactase from birth.
• Developmental lactose intolerance: Seen in premature infants whose small intestines have not yet fully matured.

Why Is Lactose Intolerance Tested?

Testing for lactose intolerance helps identify the cause of digestive symptoms and guides dietary management:

• Chronic digestive symptoms: If you experience recurrent bloating, abdominal pain, flatulence, or diarrhoea, particularly after consuming dairy products, testing can confirm or rule out lactose intolerance as the cause.
• Distinguishing from other conditions: The symptoms of lactose intolerance overlap significantly with irritable bowel syndrome (IBS), coeliac disease, inflammatory bowel disease, small intestinal bacterial overgrowth (SIBO), and cow's milk protein allergy. Objective testing helps differentiate between these conditions.
• Guiding dietary advice: A confirmed diagnosis allows targeted dietary modification - reducing lactose intake rather than unnecessarily eliminating all dairy, which could compromise calcium and vitamin D intake.
• Identifying secondary causes: If lactose intolerance develops suddenly in an adult who previously tolerated dairy, it may indicate an underlying gastrointestinal condition (such as coeliac disease or Crohn's disease) that requires investigation and treatment.
• Genetic testing for primary lactose intolerance: A blood test analysing the LCT gene (specifically the C/T-13910 polymorphism) can determine whether you carry the genetic variant for lactase persistence or non-persistence, predicting your likelihood of primary lactose intolerance.
• Children's health: In children with failure to thrive, chronic diarrhoea, or abdominal distension, lactose intolerance testing helps guide nutritional management without unnecessarily restricting dairy intake during critical growth periods.

Normal Ranges

Several tests are used to diagnose lactose intolerance. The most common are the hydrogen breath test, the lactose tolerance blood test, and genetic testing.

Lactose Tolerance Blood Test

Measurement	Normal (Lactase Persistent)	Abnormal (Lactose Intolerant)	Units
Blood glucose rise after 50g lactose load	> 1.1 mmol/L rise above baseline	< 1.1 mmol/L rise above baseline	mmol/L
Peak blood glucose timing	Within 60-120 minutes	Flat or minimal rise	minutes

Hydrogen Breath Test

Measurement	Normal	Positive for Lactose Malabsorption	Units
Hydrogen rise above baseline	< 20 ppm	≥ 20 ppm	parts per million (ppm)

Genetic Test (LCT C/T-13910 Polymorphism)

Genotype	Interpretation	Clinical Significance
C/C	Lactase non-persistent	Genetically predisposed to primary lactose intolerance
C/T	Lactase persistent (heterozygous)	Likely to maintain lactase production into adulthood
T/T	Lactase persistent (homozygous)	Will maintain lactase production into adulthood

The genetic test is particularly useful because it provides a definitive answer about primary lactose intolerance without the discomfort and time commitment of a breath test or lactose load. However, it cannot diagnose secondary lactose intolerance, which is caused by intestinal damage rather than genetics.

What Do Positive Lactose Intolerance Results Mean?

A positive result (indicating lactose malabsorption or lactase non-persistence) has several implications:

• Primary lactose intolerance: If genetic testing shows the C/C genotype, you are genetically predisposed to reduced lactase production. This is the most common worldwide pattern and is not a disease - it is the normal state for the majority of humans.
• Lactose malabsorption confirmed: A positive breath test or abnormal lactose tolerance test confirms that your body is not effectively digesting lactose. However, the degree of symptoms varies greatly between individuals - some people with lactose malabsorption remain asymptomatic.
• Symptom correlation: If your digestive symptoms coincide with dairy consumption and testing is positive, lactose intolerance is very likely the cause. Dietary modification should provide significant relief.
• Possible secondary cause: If you previously tolerated dairy without problems, a positive result may indicate secondary lactose intolerance caused by intestinal damage. Further investigation for coeliac disease, Crohn's disease, or other gastrointestinal conditions may be warranted.
• Dose-dependent tolerance: Most people with lactose intolerance can tolerate some lactose (typically 12-15g, equivalent to a glass of milk) without symptoms, particularly when consumed with other foods. Total dairy avoidance is rarely necessary.
• Nutritional considerations: Dairy is an important source of calcium, vitamin D, iodine, and protein in the UK diet. If you reduce dairy intake, alternative sources or supplementation may be needed to prevent deficiency.

What Do Negative Lactose Intolerance Results Mean?

A negative result (indicating lactase persistence or normal lactose digestion) suggests:

• Lactose intolerance unlikely: Your body produces adequate lactase to digest normal amounts of lactose. Your digestive symptoms are likely caused by another condition.
• Consider other diagnoses: If dairy still seems to trigger symptoms despite negative lactose intolerance testing, other possibilities include cow's milk protein allergy (IgE or non-IgE mediated), IBS, SIBO, fructose malabsorption, or sensitivity to other components in dairy products.
• IBS: The symptoms of IBS and lactose intolerance overlap considerably. NICE guidelines for IBS recommend dietary and lifestyle modifications, including the low FODMAP diet (which includes lactose restriction) as a second-line approach.
• Functional dyspepsia: Non-specific digestive discomfort after meals may be functional rather than related to a specific food intolerance.
• Psychological factors: Nocebo effects (experiencing symptoms because you expect them) are well-documented in food intolerance studies. Blinded challenge tests can help distinguish true intolerance from perceived intolerance.
• False negative: Breath tests can produce false negatives if the patient has recently taken antibiotics (which alter gut flora), or if the patient is a "non-hydrogen producer" who produces methane instead. Combined hydrogen-methane breath testing improves sensitivity.

How to Manage Lactose Intolerance

Lactose intolerance is highly manageable with appropriate dietary strategies. The goal is symptom control whilst maintaining adequate nutrition:

Dietary Strategies

• Identify your tolerance threshold: Most people with lactose intolerance can handle 12-15g of lactose per day (roughly equivalent to 250ml of milk) without significant symptoms, especially when spread across meals. Start with small amounts and gradually increase to find your personal threshold.
• Choose naturally low-lactose dairy: Hard and aged cheeses (Cheddar, Parmesan, Gruyere) contain very little lactose due to the ageing process. Butter also has minimal lactose. Greek yoghurt and other fermented dairy products contain bacterial lactase that aids digestion.
• Use lactose-free alternatives: Lactose-free milk and dairy products (such as Lactofree brand) are widely available in UK supermarkets. These contain the same nutrients as regular dairy but with lactase enzyme added to pre-digest the lactose.
• Try plant-based alternatives: Fortified soya, oat, almond, and coconut milks can substitute for cow's milk. Choose products fortified with calcium (at least 120mg per 100ml) and vitamin D to match the nutritional profile of dairy.
• Consume dairy with other foods: Eating lactose-containing foods as part of a mixed meal slows gastric emptying and allows more time for whatever lactase you do produce to work, reducing symptoms.

Lactase Supplements

• Over-the-counter lactase tablets: Products containing lactase enzyme (available in pharmacies and health food shops) can be taken just before consuming dairy. They provide supplemental lactase to help digest the lactose before it reaches the large intestine.
• Lactase drops: These can be added to milk 24 hours before consumption to pre-digest the lactose.

Nutritional Safeguards

• Ensure adequate calcium: The NHS recommends 700mg of calcium daily for adults. If dairy intake is significantly reduced, ensure calcium from other sources: fortified plant milks, tinned fish with bones (sardines, salmon), tofu set with calcium, green leafy vegetables (kale, broccoli - but not spinach, as oxalates inhibit calcium absorption), and calcium-fortified bread.
• Monitor vitamin D: Dairy is a source of vitamin D in the UK diet. Public Health England recommends a daily supplement of 10 micrograms (400 IU) of vitamin D for all UK adults during autumn and winter, and year-round for those with limited sun exposure.
• Check iodine intake: Milk is the main source of iodine in the UK diet. If you switch to plant-based alternatives, ensure they are iodine-fortified or consider a supplement, particularly if you are pregnant or breastfeeding.

Addressing Secondary Lactose Intolerance

• Treat the underlying condition: If coeliac disease, Crohn's disease, or another condition is damaging your intestinal lining, treating the underlying cause often restores lactase production. Once the intestinal villi recover, dairy tolerance typically returns.
• Post-gastroenteritis: Temporary lactose intolerance following a bout of gastroenteritis usually resolves within 2-4 weeks as the intestinal lining heals.

When Should You Get Tested?

Consider lactose intolerance testing if you experience:

• Bloating, abdominal cramps, or distension after consuming milk or dairy products
• Flatulence or rumbling stomach noises (borborygmi) that worsen with dairy consumption
• Diarrhoea or loose stools within 30 minutes to 2 hours of eating dairy
• Nausea after drinking milk or eating ice cream
• Chronic digestive symptoms that have not been explained by other investigations
• You are of East Asian, African, South Asian, or Mediterranean descent (higher prevalence groups)
• You developed new dairy-related symptoms after a gastrointestinal illness, coeliac disease diagnosis, or intestinal surgery
• A child in your care has chronic diarrhoea, abdominal pain, or failure to thrive that may be related to dairy intake

It is important not to self-diagnose lactose intolerance and unnecessarily eliminate dairy without testing, as this can lead to inadequate calcium intake and increased risk of osteoporosis, particularly in women. A proper diagnosis ensures appropriate dietary advice and identifies any underlying conditions that may need treatment.

Which Lola Health Tests Include Lactose Intolerance?

Lola Health offers comprehensive blood testing that can help you investigate digestive concerns and assess your overall gut and metabolic health:

• Peak Insights - Our most detailed health panel, covering a wide range of biomarkers including inflammatory markers, nutritional status, and metabolic health to help identify the root causes of digestive symptoms.
• Vital Check — A detailed wellness assessment including key markers of nutritional health, inflammation, and metabolic function.
• Core Health — An essential health check covering foundational biomarkers to provide a clear picture of your overall wellbeing and help guide further investigation.

All Lola Health tests use venous blood draws performed by qualified phlebotomists for medical-grade accuracy. Results are reviewed by GP-certified professionals and delivered with personalised health recommendations tailored to your needs.