A healthy adult, by definition, shouldn’t experience spontaneous bruising. If you mean an apparently healthy adult, then there are numerous hereditary and acquired disorders which cause spontaneous bruising despite normal PT/INR and platelet count (PC).
A frequently asked question is which conditions can cause bleeding tendency despite normal PC, INR and PTT (these are generally called screening tests for bleeding tendency). If you really meant normal PC and PT/INR, however, add factor VIII, factor IX and factor XI deficiency to the following list, as these are all associated with normal PC, INR and PTT.
Excluding the drug-induced platelet function defects and acquired disorders that interfere with platelet function (uremia, dysproteinemias…), the most common condition associated with a normal screening test for bleeding tendency is von Willebrand’s disease (vWD). Except for some severe forms of vWD, which may show mild prolongation of PTT, the majority of patients with vWD have normal PC, INR and PTT. Depending on the sensitivity of PTT reagents to deficiencies of factor VIII and factor IX, some individuals with mild hemophilia or factor IX deficiency may also have normal PC, INR and PTT. Consult the chart below for other conditions.
Conditions associated with easy bruising but normal PC, INR and PTT
I. Platelet function defects:
Hereditary: most cases of von Willebrand Disease (vWD), Glanzman’s thrombasthenia, Bernard Soulier syndrome, gray platelet syndrome, storage poor defect
a) drug induced platelet dysfunction: aspirin, chlopidogrel, some analgesics, antibiotics, cardiovascular drugs and psychotropic drugs
c) disorders of hemopoietic system: paraproteinemias, myelodysplasias, myeloproliferative disorders
II. Disorders of vascular or perivascular tissues:
Hereditary: hereditary hemorrhagic tellangiectasia, Ehler Danlos disease, osteogenesis imperfecta, pseudoxanthoma elasticum, Marfan syndrome
c) vasculitis: Henoch-Schönlin purpura, urticarial vasculitis
d senile purpura
e) others: steroid induced purpura, cryoglobulinemia, hyperglobulinemic purpura, psychogenic purpura
III. Disorders of blood clotting:Factor XIII deficiency, most cases of vWD, some of mild factor VIII and factor IX deficiency (depending on the sensitivity of PTT reagent to factor VIII and factor IX)