Why one marker rarely tells the story: iron as an example
By Alina Radu, founder of Sera · November 10, 2026
A ferritin result read on its own can mislead in either direction. Ferritin is an acute-phase protein, so inflammation raises it independently of iron stores, and a normal-looking ferritin can hide a deficiency. A high ferritin means something different depending on transferrin saturation. Read ferritin with transferrin saturation, serum iron, TIBC, hemoglobin and hsCRP from the same draw, and the panel shows a picture no single marker does.
I build Sera, a lab-review workspace for practitioners, so read the last section with that in mind. Nothing here is a diagnosis or a treatment plan. It's about how the markers relate, so that a single flagged result isn't over-read, and so you know when to send the results to the client's doctor.
What each marker adds
All figures below come from Sera's range notes, and each range page lists the sources behind them.
| Marker | What it adds | Lines worth knowing |
|---|---|---|
| Ferritin | Stored iron, but raised by inflammation | ASH (2026) diagnoses iron deficiency in adults at 30 ng/mL or lower, and 50 or lower in higher-risk groups such as people with heavy menstrual bleeding. The WHO line of 15 is lower and more specific. |
| Transferrin saturation | The share of the iron transport protein carrying iron | Below 20% is where heart failure and kidney guidelines count iron as deficient. Above 45% in women, and above 50% in men and postmenopausal women, is where the European liver guideline starts to suspect overload. |
| Serum iron | Iron in circulation at the moment of the draw | MedlinePlus gives 59 to 158 µg/dL in men and 37 to 145 in women. It varies through the day and falls with infection and inflammation. |
| TIBC | How much iron the blood's transport protein could carry | Laboratories agree on a lower limit near 240 to 250 µg/dL and differ at the top, from 400 to 450. |
| Hemoglobin | Whether the client meets the definition of anemia | The WHO defines anemia as below 12 g/dL in non-pregnant women and below 13 g/dL in men. |
| hsCRP | Whether inflammation may be distorting the other numbers | Below 1 mg/L is low cardiovascular risk, 1 to 3 average, above 3 high. A single raised result can reflect any recent infection or injury. |
Narrower optimal bands exist for most of these, and they're targets, not thresholds. Sera's notes say so: the optimal ranges for serum iron, TIBC and hemoglobin have no published source, and the transferrin saturation band of 25 to 35% is a functional target that a Danish cohort study supports without setting its edges.
Why ferritin alone misleads
A low ferritin is the clearest single result. At 30 ng/mL or lower, ASH diagnoses iron deficiency in adults. But hemoglobin can still be inside its range at that point. Read hemoglobin alone and the low iron is missed. Read ferritin alone and you don't know whether hemoglobin has fallen too.
A normal ferritin isn't always reassuring. Because inflammation lifts ferritin, ASH considers iron deficiency possible up to 100 ng/mL when inflammation is present, and in that setting it reads a transferrin saturation under 20% instead. That's why an hsCRP from the same draw matters. Without it, a ferritin of 70 looks fine and there's no way to tell whether inflammation is holding it up.
A high ferritin needs its partner. The European liver guideline prompts a check for iron overload when ferritin is above 200 in women or above 300 in men and transferrin saturation is also raised. A high ferritin with a normal saturation is a different picture, and inflammation is one of the things that can produce it.
Serum iron moves too much to read alone. It varies through the day and drops with infection, so one low serum iron from an afternoon draw during a cold says very little. Read it with ferritin and saturation, which is what its range note recommends.
Patterns, not single flags
When the markers are read together, a few recognized combinations stand out. Sera points out these patterns across a client's results. They're observations about how the numbers sit together, not diagnoses, and the client's doctor is the one to interpret them.
- Low ferritin with hemoglobin still in range. Sera calls this pattern Iron Deficiency Without Anemia, the name the literature uses for that stage. Neither marker alone shows it.
- Low saturation, with ferritin not low and hsCRP raised. Sera calls this Low Iron Availability with Inflammation. Here the ferritin is the misleading number: it reads normal while the saturation doesn't.
- Raised saturation with raised ferritin. Sera calls this Raised Transferrin Saturation with Raised Ferritin. It's the combination the European liver guideline uses to prompt testing for overload, and a reason to refer.
Each pattern needs more than one marker, which is the point. A practitioner who only ordered ferritin can't see any of them.
Conditions that change the picture
Several of these markers move for reasons unrelated to iron stores. Before reading the panel, check:
- Fasting and time of day. Transferrin saturation needs a fasting morning sample, because it follows serum iron through the day.
- Recent illness. A single raised hsCRP can reflect any recent infection or injury, ferritin rises with inflammation, and serum iron falls with it. The hsCRP note says to repeat the measurement when the person is well, and the same logic applies to reading the iron markers from that draw.
- The same draw. Read markers from one blood draw together. A ferritin from March and a saturation from June describe two different days.
- Things that shift hemoglobin. Altitude, smoking and plasma volume all shift hemoglobin independently of iron status.
- The client's sex. Bands for hemoglobin, transferrin saturation, ferritin and serum iron differ by sex. Record it, or a result can be read against the wrong line.
The general habit
Iron is the clearest example, but the habit applies widely: read a marker with the markers it depends on, from the same draw, before reading it against a range. TSH reads better next to free T4, as Reading thyroid labs over time explains. A single flagged result is a reason to check its partners and the conditions of the draw, and often to repeat it, before it's a reason to conclude anything. And a series of panels shows more than one, which Reading a client's blood work over years covers.
When to refer
A low hemoglobin, a raised saturation with a raised ferritin, a ferritin that keeps falling, or any iron result outside the lab's own range belongs with the client's doctor. Send the full panel with the date of the draw and any notes on illness, fasting and medications, so the results can be read in context.
Where Sera fits
Sera points out recognized patterns across markers, including the three iron patterns above, and lets you put up to three markers on one chart, so ferritin, transferrin saturation and hsCRP can be read together over years. Every band has its sources and evidence grade published, so you can see that the ferritin floor is an established clinical threshold and the serum iron and TIBC bands are published reference intervals. Results stay in the unit the lab reported, diet, supplement and medication changes are marked on the trend charts, and Sera suggests a retest date for any result out of range. It doesn't diagnose or recommend treatment.
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Alina Radu is the founder of Sera and a software engineer. The idea started with her own Hashimoto's tests, scattered across years without the big picture.