Bruises without a cause

In shortBruises without injury and a bleeding tendency are worked up in stages: first a complete blood count with platelets and a smear, then the basic coagulation panel (INR, APTT, fibrinogen), then the extended block — thrombin time, antithrombin III, lupus anticoagulant and, when needed, factor assays. Each step rules out its own group of causes.

Step one: platelets and the smear

The first question is whether there are enough platelets and whether they look normal. The complete blood count shows the platelet number, and the smear under the microscope shows their appearance and whether they are being destroyed. Immune thrombocytopenia is a frequent cause of unexplained bruising: guidelines describe it as a diagnosis of exclusion that begins exactly with the blood count and smear.

Pinpoint bruises (petechiae) on the legs, bleeding gums and nosebleeds are complaints of the platelet type. Large hematomas after trivial trauma and bleeding after procedures pull rather toward the plasma arm — the coagulation panel.

Step two: the basic coagulation panel

INR and APTT are two screens of plasma clotting: the extrinsic and intrinsic pathways. A prolonged INR points to the extrinsic-pathway factors and liver function; a prolonged APTT to the intrinsic-pathway factors, including those affected in von Willebrand disease and the hemophilias. Fibrinogen completes the picture: its shortage lengthens the final step of clotting.

An important caveat: mild von Willebrand disease — the most common inherited clotting disorder — can give a normal basic panel. A convincing bleeding history with a normal screen therefore does not close the search; it moves it to the next level.

Step three: the extended block

When the basic screen is normal but the history is convincing, the look goes deeper: thrombin time assesses the final conversion of fibrinogen to fibrin, the lupus anticoagulant is excluded as a cause of the long APTT, and antithrombin III is checked not as a cause of bruises but as the opposite pole — with a family history of thrombosis. Factor assays and von Willebrand testing belong to the hematologist.

The medication layer is always checked: antiplatelets, anticoagulants, some antibiotics and even supplements change clotting. The list of taken agents is a mandatory part of the review — without it the report reads wrong.

When bruises are not about clotting

Part of unexplained bruising is linked not to blood but to vessels and skin: age-related capillary fragility, long-term use of certain drugs, vitamin C deficiency. These causes are visible neither in the blood count nor in the coagulation panel — they are defined by examination and history.

Warning combinations that should not be postponed: bruises plus pallor and fatigue (hemoglobin and ferritin are checked), bruises plus fever and enlarged lymph nodes, bleeding that does not stop in the usual time. Here the route speeds up and is led by a doctor.

What to write down before the doctor's visit

Where bruises appear and how fast they fade
Nose and gum bleeding: frequency and duration
Bleeding after tooth extractions and operations in the past
All medications and supplements being taken
Bruising and bleeding in relatives

Markers covered in this article

Platelets (PLT)Prothrombin time aPTT Lupus anticoagulant Antithrombin III Thrombin time Fibrinogen

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Frequently asked questions

Which tests are drawn for bruises without a cause?

First step — a complete blood count with platelets and a smear; second — the basic coagulation panel: INR, APTT, fibrinogen. Then, by results, the extended block and factor assays as the hematologist assigns.

Platelets are normal but bruises appear — what next?

The next layer is the plasma arm: INR, APTT, fibrinogen. If it too is normal with a convincing history, von Willebrand disease and vascular causes are excluded — a normal screen does not close the search.

Can medications cause bruises?

Yes: antiplatelets, anticoagulants and a number of other agents affect clotting, and some drugs affect the vessel wall. The list of taken agents is shown to the doctor together with the report.

Pinpoint bruises on the legs — is it dangerous?

Pinpoint spots (petechiae) are a reason to draw a blood count with platelets soon: thrombocytopenia can look this way. Immune thrombocytopenia guidelines begin diagnostics with exactly this step.

Is preparation needed for these tests?

Blood is drawn fasting; for the coagulation panel, proper filling of the citrate tube matters — the laboratory handles that. Anticoagulant and antiplatelet intake is reported in advance: it distorts the tests.

Nosebleeds in a child — which tests?

Most often the cause is local — a dry mucosa and fragile septal vessels. With frequent, heavy bleeds a blood count with platelets and the basic coagulation panel are drawn; the rest follows the results.

Sources

Immune thrombocytopenia — clinical guideline context — Minzdrav of Russia (cr.minzdrav.gov.ru). Accessed: August 29, 2026.
Partial thromboplastin time (PTT): MedlinePlus Medical Encyclopedia — MedlinePlus, US National Library of Medicine. Accessed: August 31, 2026.
Thrombin Time Test: What It Measures — Testing.com (Lab Tests Online, AACC). Accessed: August 31, 2026.

Bruises without a cause: the test sequence from platelets to the coagulation panel