Most people with loose stools do not have a parasite, so the useful question is what pattern you are seeing and how long it has lasted. This page walks through the everyday causes first, then the testing that separates them.
Most loose stools and bloating come from everyday causes rather than an infection, and they settle within a few days. Before anyone thinks about parasites, it is worth looking at what changed in the week or two beforehand.
The common everyday drivers include the following.
Giardiasis is one possible reason when the pattern drags on past a few days, when stools turn greasy and foul-smelling, and when there has been plausible water exposure. Giardia spreads as tough cysts shed in stool that survive for long periods in cool water and can pass through standard filtration, which is why drinking water, lake and river water, and swimming pools all appear in outbreak reports 2. Person to person spread in households also occurs, so a lack of camping trips does not rule it out 3.
It is also worth knowing that a positive stool test does not always come with symptoms. Research suggests that a sizeable share of people carrying giardia have no complaints at all, which means context and timing matter when results are interpreted 3.
The classic picture is watery or soft stools that become greasy, pale and strongly foul-smelling, paired with upper abdominal bloating, excess gas and cramping 1. Nausea, early fullness after small meals, sulphurous burping and fatigue are commonly described alongside it 4.
Two features help separate this from a run-of-the-mill stomach bug. The first is duration. A viral gut upset usually fades within a few days, whereas giardiasis symptoms often persist or come and go in waves over weeks 5. The second is the stool itself. Fat that is not absorbed makes stools bulky, pale and prone to floating, a pattern that points towards the small intestine rather than the colon 6.
High fever and visible blood in the stool are not typical of this infection, and their presence usually points a clinician towards other explanations 5. Unplanned weight loss over several weeks is reported and reflects a combination of reduced intake and reduced absorption 6.
None of this amounts to a diagnosis. Several non-infectious conditions produce an almost identical story, which is exactly why testing exists.
Symptoms can outlast the parasite because the lining of the small intestine takes time to recover. Giardia trophozoites attach to the surface of the upper small intestine, where nutrients are absorbed, and research suggests that this disturbs the epithelial barrier and the brush border enzymes that sit on it 7.
Lactase, the enzyme that breaks down the sugar in milk, lives on that brush border. When it is temporarily reduced, dairy that was previously fine can produce bloating, gas and loose stools, and studies indicate that this kind of acquired lactose intolerance can follow giardiasis and persist for a period after the infection resolves 6. For many people it improves as the lining regenerates.
There is a second pattern worth naming. Giardiasis is associated with longer-term bowel symptoms and fatigue in a subset of people after the parasite itself has gone 3. If that describes you, it is worth discussing with a clinician rather than assuming the infection is still active, because repeat courses of anything are a decision for a licensed provider who has seen your results.
Stool testing is what identifies the parasite, and blood work describes the downstream effects on your nutrition and protein status. The two answer different questions, and a blood panel alone cannot confirm or exclude this infection.
On the stool side, research suggests that antigen detection and nucleic acid amplification tests identify giardia more reliably than microscopy alone, and microscopy for cysts in a stool sample remains widely used 1. Because cysts are shed intermittently, more than one sample collected on different days is often requested 5. When stool results stay negative and symptoms persist, sampling from the upper small intestine during endoscopy is sometimes used to look directly for the organism 8.
Blood markers add the nutritional picture. The table below shows what each one contributes and where it stops.
| Marker | Why it is linked to these symptoms | What a result can and cannot tell you |
|---|---|---|
| Albumin | The main protein in blood, and levels can drift down when intake falls, absorption falls or inflammation rises. Studies indicate that malabsorption of fat and nutrients can occur during giardiasis 6 | A below-range result flags that protein status or inflammation deserves attention. It cannot say whether a parasite, a diet pattern, a liver issue or kidney protein loss is behind it |
| Haemoglobin and iron markers | Weeks of loose stools and reduced small intestine absorption can eventually show up in red cell measures | They show whether oxygen-carrying capacity and iron stores are holding up. They never identify an infection, and they can stay normal early on |
| Vitamin B12 and folate | Both are absorbed in the small intestine, the same region this parasite colonises | They show current status. They cannot date when the change started or name its cause |
Yes, it can, and that is why Albumin is one of the markers clinicians commonly check when prolonged diarrhoea and weight loss are being evaluated. Albumin is the most abundant protein in blood. It is made by the liver, it holds fluid inside blood vessels, and it carries hormones, calcium and fatty acids around the body.
The mechanism is straightforward. Giardia settles in the duodenum and upper jejunum, which is where most protein, fat and fat-soluble vitamin absorption happens, and research suggests that the resulting disruption to the epithelial surface reduces that absorptive capacity 7. Add reduced appetite, several weeks of nausea and a few kilograms of unplanned weight loss, and the raw material for albumin production drops. Studies indicate that clinically meaningful malabsorption occurs in a portion of people with giardiasis, which is the pathway by which protein status can slip 6.
| Marker | Typical lab reference range | Units |
|---|---|---|
| Albumin (adults) | 3.5 to 5.0 | g/dL |
| Albumin (adults, SI units) | 35 to 50 | g/L |
These are typical lab reference ranges. Exact cut-offs vary by lab, age and sex, so always read your own result against the range printed on your report.
Plenty of things other than an infection move albumin, which is why a single value is never a diagnosis. Dehydration concentrates the blood and can push albumin up. Acute inflammation pulls it down, because the liver shifts production towards other proteins. Liver conditions reduce how much is made, and kidney conditions can lose protein in the urine. Even posture and how long a tourniquet stayed on your arm shift the number slightly. The useful reading is observation, then possible meaning, then a conversation with a clinician who can see the rest of your panel and your history.
They describe how much the gut disturbance has cost you, not what caused it. Diagnosis of this infection rests on stool testing, so blood work is supporting information rather than the answer 1.
A full blood count shows haemoglobin and red cell size. Microcytic cells with low iron stores suggest iron has been under pressure for a while, which can happen when absorption has been impaired for weeks. Larger cells raise the question of B12 or folate status, both absorbed further along the same stretch of small intestine that this parasite occupies.
Fat-soluble vitamin status is the other quiet casualty of fat malabsorption, since vitamins A, D, E and K travel with dietary fat. None of these markers is specific. A below-range value tells you something is worth looking into and nothing more, and the next step is a clinician reading it alongside your symptoms and your stool results.
A clean blood panel does not rule out this infection, because the parasite lives in the gut lumen and never has to show up in your bloodwork. Early in an infection, albumin, haemoglobin and vitamin status are often entirely normal, and the only abnormality is in a stool sample.
Stool testing has its own blind spots. Cysts are shed in bursts rather than continuously, so a single negative sample does not settle the question when symptoms persist, and repeat samples on separate days are the standard way around it 5. Direct sampling of the upper small intestine is an option a clinician may consider when stool testing stays negative and the picture still fits 8.
No blood test sees the everyday explanations either. Sugar alcohols, a big fibre change, alcohol, poor sleep, a hard training block and plain anxiety all produce real bowel symptoms with a perfectly ordinary panel. Functional bowel disorders, coeliac disease, bile acid issues and other infections also sit behind this symptom set, and they need their own specific tests. Testing narrows the field. It rarely closes the case on its own.
Some patterns are worth discussing with a clinician promptly rather than waiting to see whether they pass. Contact a clinician without delay in the following situations.
Do not try to work out which organism you have from a symptom list, and do not take anything left over from a previous illness. Testing is cheap and specific, and treatment decisions belong to a licensed provider who has reviewed your results. If a result from any panel comes back outside the standard range, that is a reason to ask about it, never a reason to ignore it.
If you are having thoughts of self-harm, call or text 988 (US) or your local emergency number now.
The markers described here are measured on a standard baseline blood panel, and the FitScript Essential panel includes albumin alongside the blood count and nutritional markers discussed above, within a membership that covers 150+ biomarkers tested per year. Atlas, FitScript's AI health advisor, explains what each result means in plain language and shows how the markers move together over repeat draws, which is more informative than one isolated value.
Two honest caveats belong here. A blood panel does not detect this parasite, so if your history points towards water exposure and greasy stools, a stool test ordered by a clinician is the relevant investigation. And any next step, from further testing to any form of management, is decided by a licensed clinician who evaluates whether it is appropriate for you.
This article is educational and is not medical advice. Take your results and your symptom history to a clinician or licensed provider and decide together what, if anything, to do next.
Related reading on this site covers malabsorption symptoms in more detail, including the other conditions that produce greasy stools and nutrient gaps.
Giardia is a single-celled intestinal parasite that lives in the upper small intestine. It spreads through a hardy cyst form shed in stool, which survives in water and can be swallowed from drinking water, lakes, rivers, pools or close contact at home. Once swallowed, it attaches to the gut lining where nutrients are absorbed.
The most common ones are soft or watery stools that turn greasy, pale and strongly foul-smelling, along with bloating, excess gas, cramping, nausea and tiredness. Unplanned weight loss can follow if the symptoms continue for weeks. High fever and visible blood in the stool are not typical and point a clinician elsewhere.
A stool test confirms it. Laboratories use antigen detection, molecular testing or microscopy, and because the parasite is shed in bursts, your clinician may ask for samples collected on different days. If stool results stay negative and symptoms persist, sampling from the upper small intestine during endoscopy is one option a clinician may consider.
No. The parasite lives in the gut and does not show up directly on a routine blood panel. Blood markers such as albumin, haemoglobin, iron, B12 and folate describe the nutritional effect of weeks of poor absorption, which is useful context. Identifying the organism itself still requires a stool sample.
The enzyme that digests milk sugar sits on the surface of the small intestine lining, which is exactly the area affected. When that surface is disturbed, lactose passes through undigested and ferments, producing bloating, gas and loose stools. For most adults it improves as the lining recovers. Mention it to your clinician if it persists.
Both are possible routes. The cyst form survives for long periods in cool water and resists some standard filtration, so drinking water outbreaks, lake and river water, and swimming pools all appear in public health reports. Household spread from person to person also happens, so a recent camping trip is not required.
Contact a clinician and describe your symptoms, how long they have lasted and any water exposure, then follow their testing advice. Keep fluids up in the meantime. Do not take leftover medicine from a previous illness or self-prescribe anything. Any decision about management belongs to a licensed provider who has reviewed your results.
Seek care promptly for signs of dehydration such as dizziness on standing or passing very little urine, for blood in the stool or black stools, for a high fever, for severe or worsening abdominal pain, or for diarrhoea lasting more than a few days. Unplanned weight loss also warrants a prompt review.